FAQ :  What are opal fossils? Where are black opal fossils found? What does opalized / opalised mean?

Lightning Ridge – Black Opal Fossils

Only one place on Earth produces black opal fossils – Lightning Ridge in northern New South Wales, Australia. Lightning Ridge is the only opal field in Australia with fossils of diverse land-living organisms – pinecones and platypuses, microscopic protozoans and gigantic dinosaurs. The fossils are usually exact replicas of plant, shell or bone material, and at times they are comprised of gem quality black opal, which is as valuable as diamonds and more beautiful. 

Black opal fossils which may be found at Lightning Ridge include remnants of ancient plants, mussels, snails, crustaceans, fish, turtles, plesiosaurs, crocodiles, pterosaurs, dinosaurs, birds, and mammals. 110 million years ago the supercontinent Gondwana was a wilderness of forests of pines, ferns and palms separated by tracts of shallow sea. Dinosaurs and their relatives dominated this landscape, as well as our rare and tiny mammal ancestors. Near the edge of this ancient continent, fragments of the remains of these animals accumulated in the sands of the inland sea. Today deposits at Lightning Ridge in northern New South Wales yield some of the rarest, most beautiful and precious fossils in the world.

Lightning Ridge fossils are three-dimensional replicas of ancient organic objects, transposed into non-precious potch or precious opal. In those that are pseudomorphs, the silica has filled a simple cavity or void, like jelly in a mould, so that only the basic shape and perhaps the surface texture is preserved. However, many specimens are replacement fossils, in which intricate internal structures have been preserved by chemical alteration before the cavity was filled by the silica solution. 

   

Most specimens at the Ridge are a combination of pseudomorph and replacement fossils. Although the transformation to silica has destroyed biomolecular evidence, marrow tissue, blood vessels, capillaries and nerve channels may be perfectly preserved. If the potch is transparent, these features are clearly visible below the surface in opalised bones. A surprising aspect is the opalisation of delicate materials like leaves and even dinosaur skin. Many pieces resemble coprolites, reptilian armour scutes or heavy scales; very occasionally, bone specimens seem to show remnants of tendons or cartilage. 

This outstanding quality of preservation is partly because the opal-dirt is extremely fine-grained and an ideal casting medium. Kaolinite, smectite, and illite produce the putty-like properties of the opal clay, the smectite making it plastic and malleable. 

Most opal fossils found at the Ridge consist of potch (colourless opal), therefore any fossils with colour are rare and valuable. Many fossils are damaged by machinery during excavation, as pick and shovel based operations are giving way to machine-driven excavations. Removing fossil specimens can be a delicate operation, and colourless fossil specimens are largely ignored by miners searching for colour.

South Australian Fossils

The opal fossils of the South Australian opal fields are both jewels of science and beautiful gems. The Eromanga Sea that covered the interior of Australia 100-120 million years ago was rich in marine life. Ichthyosaurs, plesiosaurs, fish, sharks, ammonites and belemnites swam in the open water. Slow-moving and sedentary animals, such as starfish, crinoids, cockles, mussels, snails and tube-worms lived on the seafloor.

Only those bones and shells that became trapped in seafloor sediment had a chance of becoming fossils. Some were replaced by clear silica, and others by precious opal. 

One area which yields a vast resource of opal fossils is Moon Plain, approximately 35km from Coober Pedy in South Australia’s outback. Once part of a vast inland sea which covered most of Australia, Moon Plain once teemed with marine life, but is now a wonderland for palaeontologists. South Australia has the best cold water cretaceous marine deposits in the world, and it is difficult to imagine the extreme freezing temperatures of millions of years ago compared with today’s scorching heat.

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Safety on the opal fields

Australia produces over 90 per cent of the world’s precious opal, and more than half comes from NSW opal fields. The major opal-producing areas in NSW are Lightning Ridge and White Cliffs. Visitors may fossick or “spec” for opal on a field provided they have first obtained permission of the landowner or leaseholder. If the area is subject to a registered mineral claim, a visitor must also seek permission from the claimholder. Please see section below on “The Mining Act 1992”.

 

A warning

A fifteen year old boy was helping in his father’s underground opal mine when he fell six metres down an internal vertical shaft and suffered a broken leg and bruised back. He was helping his uncle and father who were mining an exploration drive about 10 metres below the surface. They had deliberately holed into the small old workings of the adjacent mine so that they could evaluate the ground ahead of them. They had one porta flood light between them that was powered by a surface generator. The electric cable to their light only stretched two metres into the old workings, but they saw what they wanted to see and the two adults returned through the opening. Unknown to the adults, the son, who was at the rear, suddenly decided to explore one of the old drives, even though he had no light to see where he was going. In the darkness, he stepped into a small unprotected six metre deep vertical internal shaft in the drive floor and fell to the bottom. He was immobilised by his injury. The rescuers gained access to where he was through an adjoining shaft.

Please exercise extreme caution whilst mining or visiting a mining field;

  • Do not allow inexperienced people alone in underground mine workings
  • All persons who are taken underground must be instructed and/or supervised to an appropriate level of understanding.
  • Every person underground should carry a light.
  • Unfamiliar workings must be considered to be dangerous and must be secured against casual entry.
  • Never position a winze in the middle of a drive or cross-cut.
  • School children on holidays in a work situation need constant supervision.

 

Fossick in Safety

Established opal fields may be good places for fossicking, but there are many hidden dangers. Children are especially vulnerable, and need to be kept under close supervision at all times. Fossickers should first make their presence known to any miners in the area. Remember heavy mining and earthmoving equipment is used on opal fields, and can operate without warning. Self-tippers and blowers bring dirt to the surface of claims from underground , often working unattended.

Shafts, drill holes and open cuts may pose a serious danger to young children. Drill holes in particular – about the width of a child’s foot – are extremely dangerous. Shafts and drill holes may be full of water and may be camouflaged by vegetation growth. Old shafts or drill holes may be in a dangerous state, with collapsed collars or protected only by rotten timber or rusted sheets of iron.

Fossickers are warned against entering old underground workings without the benefit of experienced and competent miners to assist. Many dangers exist such as unstable ground, stale air and lack of oxygen, unprotected openings underneath and holes full of water. Seek help before entering old mines. Fossicking may only take place to a depth of one metre and all holes dug must be refilled. Fossickers are also warned against entering old open cuts or costains and especially warned against fossicking into the sides of excavations which may cave in unexpectedly. Large stockpiles of dirt may also collapse unexpectedly particularly if they are undercut.

When fossicking wear strong clothing, a hat and shoes. If you are fossicking alone tell someone where you are going in case you get into difficulties. In hot weather carry plenty of water. A first aid kit is always advisable. Avoid snakes; don’t try to handle or kill them. Please note too that transport can be a problem in these areas, not only because of the distances involved, but also because of the heat. A car will be needed to visit the main fields.

 

Equipment for Fossicking

You will need, as a minimum:

  • Pick;
  • Shovels;
  • Pans;
  • Sieves;
  • A pair of tweezers and a hand lens (x8 or x10) to help identify your find;
  • Hat;
  • Insect repellent;
  • Boots;
  • Small collecting jars.

 

The Mining Act 1992

Under section 12 of the Mining Act 1992, “A person must not fossick for minerals on any land that is the subject of an authority, a mineral claim or an opal prospecting licence except wih the consent of the holder of the authority, claim or licence.”

Fossickers must not use any explosives or power-operated equipment, drill or excavate to a depth of more than one metre, damage or remove any bushrock, or remove more than 20 grams of gemstones during any single period of 48 hours. A person who wishes to do more than fossicking, for example prospect and/or mine for opal, must have a licence under the Mining Act 1992. This can be an opal prospecting licence, an exploration licence, a mining lease or a mineral claim.

In traditional opal mining areas in NSW such as in Lightning Ridge and White Cliffs, prospecting for opal is mostly carried out under a mineral claim or an opal prospecting licence, if such is available. Other States may have different requirements. The maximum area allowed in NSW under a mineral claim is two hectares but within the Lightning Ridge Mineral Claims District and the White Cliffs Mineral Claims District, where special rules apply, a mineral claim is restricted to 1/4 of a hectare (i.e. 2,500 square metres).

A mineral claim authorises the holder to prospect as well as mine and a person is restricted to two claims in each District. Opal Prospecting Licences are granted over much larger areas than those granted under mineral claims but are purely for prospecting and do not authorise mining. An Opal Prospecting Licence can only be granted over lands defined as an “Opal Prospecting Block” within an area designated under the Mining Act as an “Opal Prospecting Area”. There are a number of Opal Prospecting Areas in the Lightning Ridge and White Cliffs Mineral Claims Districts.

For further information on the various titles available, the rights and duties of holders of titles and the policies that apply, contact the Department of Mineral Resources or the Mining Registrar at Lightning Ridge, or the Mining Registrar at Broken Hill.

 

Code of Conduct

The New South Wales opal fields are located on pastoral leases, which are still used for farming purposes. Please recognise that while visiting the fields you are a guest on a working cattle or sheep station, and respect the owner’s requirements. Do not take dogs to the fields, leave all gates as you find them, and take away your garbage. Visitors are requested to read a copy of the full code of conduct, which can be obtained from Department of Mineral Resources offices. 

 

For more Information

Further reading:

  • Opals in New South Wales, Department of Mineral Resources.
  • The Mining Act 1992 (New South Wales)
  • Mine Health and Safety Act 2004 (New South Wales)
  • Code of conduct on the opal fields, Department of Mineral Resources.
  • Gemstones in Australia, Australian Gemstone Industry Council, 1993.
  • Australian precious opal: A guide book for professionals, Andrew Cody Pty Ltd, 1991.
  • Australian opals and gemstones, Australian Gem Industry Association, 1987.

 

Places to go for information

The Department of Mineral Resources
Information Counter
29-57 Christie Street 
St Leonards 2065
(PO Box 536, St Leonards NSW 1590)

Telephone: (02) 9901 8269 begin_of_the_skype_highlighting              (02) 9901 8269      end_of_the_skype_highlighting
Fax: (02) 9901 8247

OR 

Lot 60
Morilla Street
(PO Box 314)
Lightning Ridge 2834

Telephone: (02) 6829 0678 begin_of_the_skype_highlighting              (02) 6829 0678      end_of_the_skype_highlighting / 0824
Fax: (02) 6829 0825

 

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FAQ: Which is better, black opal or boulder opal? What is black opal? What is boulder opal? What is the difference between black opal and boulder opal? How are black opals valued? How are boulder opals valued?

Black opal has long been the most prized and most famous opal, however it’s lesser known cousin, the boulder opal, is beginning to gain recognition and popularity amongst opal lovers and the general public. So let’s compare these two types of opal and see how they stack up!

A Brief Introduction

Black opal was discovered in Australia at Lightning Ridge, New South Wales, in about the 1880’s or 1890’s and has since become famous around the world as the most brilliant and stunning opal in the world. It is highly sought after and prized around the world. Black opal is generally mined in shafts, and is usually found between 25-45 feet in ‘nobby’ (nodules of potch and colour) or ‘seam’ (horizontal deposit) formations. Black opal is considered the ‘Rolls Royce’ of opals as its dark body tone lends its colour an extra vibrancy not seen in the more common white opals found in South Australia.

Boulder opal was discovered in Australia at Quilpie, in Western Queensland, in about 1870. Since then it has risen to recognition as the second most valuable type of opal, however remains largely unknown to the general international public. Boulder opal is generally mined in open cut operations, where large amounts of dirt are cut away from the surface and removed. The ironstone boulders which contain the opal are referred to as ‘floating boulders’ due to their irregular positioning under the surface. The opal usually forms as thin veins within these boulders, and most stones are cut with the host ironstone still remaining on the back.

Body Tone

Black Opal – Black opal gets its name from its dark body tone, which ranges from dark grey to black. The darkness of the stone causes the colour to stand out much more than if the stone was white. Black opal often has a natural potch backing (colourless opal) on the back of the stone which also contributes to its dark body tone. (Note: triplets & doublets, which consist of a thin slice of opal glued to a black backing, are made to imitate black opals.)

Boulder Opal – Boulder opal also has a dark body tone (although there are some occurrences of white boulder opal), which is as dark as black opal. Because boulder opal forms in thin veins in ironstone boulders, the host ironstone is usually left on the back of the stone. This is why, if you look at the back of a boulder opal, there is a layer of brown rock attached to the back. For this reason, boulder opal is sometimes referred to as a ‘natural doublet’, as this layer of dark stone on the back gives the opal its dark body tone.

 

Colour & Pattern

Black Opal – Black opal may display all the brilliant colours of the rainbow, in all the patterns which opal displays, such as floral, ribbon, harlequin, straw, chinese writing, rolling flash, etc.

Boulder Opal – Boulder opal is identical to black opal in its spectrum of colours, its potential for brilliance, and in the patterns which it may display. Some argue that because boulder opal is a thin layer of opal located very close to a dark backing, it has the potential for brighter & better colour, however this is debatable.

Note: Gem quality stones (gem quality refers to extremely high quality opals) – In proportion to the amount of opal mined, there is very little difference in the quantity of rare, high quality stones which are found of each type of opal.

 

Stone Shape

Black Opal – The market for black opal generally demands symmetrical, oval shaped stones, with a domed cabochon, as these are the most popular shape in most jewellery.

Boulder Opal – Due to the thin nature of the opal veins which form in boulder opals, it is impossible to cut domed cabochons in most boulder opal stones. The stones are therefore usually cut into free form, irregular shapes (given the odd exception) to maximise the size of the stone and minimise the loss of opal. Boulder opals therefore often cater to a slightly different market and appeal to those who like irregular shapes in jewellery.

 

Strength

Black Opal – Black opal, along with white opal, is classed as having a hardness of 5.5 to 6.5 on Moh’s scale of hardness. (Diamonds being 10) Therefore it is relatively fragile stone, with a similar hardness to glass.

Boulder Opal – The opal layer in boulder opal is the same as above, however the ironstone backing which is naturally attached to the stone gives boulder opals an extra hardness and strength which gives it an advantage over other types of opal. Therefore a boulder opal will generally stand up better to impact, and be much less likely to crack than a black opal.

Rarity

Black Opal – Black opal is the rarest form of opal, and is only found in opal mining fields approximately within a 70 kilometre radius of the town of Lightning Ridge. Black opal is becoming increasingly rare and top grade black opal is not currently found anywhere else in Australia or in the world. (Unlike diamonds, which are in fact very common, black opals are genuinely rare.)

Boulder Opal – Boulder opal is currently much more readily available than black opal. The area which has the potential to yield boulder opal is much larger, and a relatively small proportion of this has been explored. The area stretches along a 200 to 300 kilometre strata in Western Queensland. Therefore, the future looks promising for boulder opal mining, as it is likely to become the only actively producing source of natural dark opal. (This is partly speculation of course).

 

Price

Black Opal – Because of its rarity and status, black opal carries with it a certain price attachment. Market forces determine that rare items which are highly sought-after fetch higher prices than more common items. Because of their beauty, rarity, and status, black opals fetch a much higher price in comparison to boulder opals. Some argue that black opals are overpriced, while others state simply that market forces (supply and demand) determine their value.

Boulder Opal – Boulder opals are relatively under-priced in comparison to black opals. A high quality, predominantly red black opal, which is identical to a boulder opal, may fetch prices up to seven times that of the boulder opal. Generally speaking though, boulder opals are considered to carry one third of the price of black opals, despite the fact that the cost of mining boulder opals is much more than that of black opals. Open cut mining requires much more machinery and fuel than shaft mining.

The reasons for this are as follows;

  • Rarity – as stated before, boulder opals are found more commonly than black opals.
     
  • Fame & Status – Black opals are renowned throughout the world for their rarity and beauty.
     
  • The Ironstone Factor – Due to the natural ironstone backing and the thin nature of the opal layer, boulder opal is not traditionally priced ‘per carat’. Because ironstone is much heavier than opal, valuers consider that inclusion of the ironstone would be a distortion to a ‘per carat’ price, and therefore a lower value is applied to the stone overall. Boulder opal therefore has different valuing standards applied to it, and is sometimes valued ‘per piece’ rather than at a price per carat.  It’s important to note, however, that black opals which have a colourless black potch backing are still valued per carat, and do not receive any kind of ‘penalty’. Practically speaking, it makes little difference what is on the back of a stone once it is set into jewellery – colour and brightness are still the most important factors.

 

Conclusion

So there you have it. Hopefully now you’ll have an understanding of the difference between black opal and boulder opal, what makes them different, and why sometimes their value differs. It’s all part of the education process!

So which is better? The truth is – neither is better, they are both equal in quality. It all depends on the individual stone, your individual tastes, and what you value in a stone. If you value the rarity of the type of stone (the satisfaction of owning something rare), the high profile often associated with it (e.g. pink diamonds or a Rolls Royce), or an oval, dome shaped opal (as opposed to a freeshape stone), then you would probably prefer a black opal over a boulder opal. More often than not, it won’t matter what type of opal it is, once you see the one for you, you’ll fall in love with it!

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A new era for opal nomenclature

Anthony Smallwood FGAA, GG
Chairman, G.A.A. Opal Nomenclature Sub-committee

 

Abstract

Opal is a relatively common mineral species that is found in many locations world wide. For many years a reason for the spectacular phenomenon known as play-of-colour, as seen in precious opal, remained a well hidden secret. It was not until the 1960s that Australian scientists working at the Commonwealth Scientific and Industrial Research Organisation (CSIRO) used a new instrument now known as the electron microscope to reveal the inner structure of opal and how this is responsible for generating the play of colour of precious opal.

Opal often was referred to as a ‘semi-precious gemstone’, until unique Australian black opal was discovered and successfully marketed. Today, all varieties of precious opal, which are mined in the Australian states of New South Wales, South Australia and Queensland, support a $A500 million per year industry.

One problem that the opal industry has been required to face, however, is how does one describe a gemstone that occurs both with and without a play-of-colour, in almost every colour of the rainbow, in every tone of lightness and darkness from black to white, and in every degree of transparency from opaque to perfectly transparent. Also this unique gemstone displays differences in mineralogy that reflect the varying geological environments in which it forms.

The opal nomenclature that follows is a result of three years of striving within Australia’s opal industry to achieve co-operation and to formulate a nomenclature for opal which is accepted uniformly throughout the industry.

 

Introduction

For many years the terminology and nomenclature used to describe opal has been widely discussed and debated by gemmologists and those members of the gem and jewellery industry who have an interest in this gemstone. Aspects of this long-running discussion can be seen in the long list of papers published throughout the forty year history of The Australian Gemmologist. But, how to best describe opal (arguably the most beautiful of gemstones) has been a contentious and difficult issue for a very long time — and may well remain so for some time to come. However, as a consequence of factors such as: growing international and local awareness of opal as a major Australian resource; the emergence world-wide of a real desire to standardise all terminology related to gemstones; and the ever growing number of synthetics and imitations that are appearing in world markets; it has became necessary to agree on some well based concepts of how a unique gem material, such as opal, should be described. It was late in 1993 that the Australian Gemstone Industry Council requested the then President of The Gemmological Association of Australia (GAA), Grahame Brown, to initiate investigations into the possibility of establishing a uniformly accepted nomenclature for opal. After a short time, a working sub-committee of the GAA was formed that consisted of representatives of The Gemmological Association of Australia, the Australian Gem Industry Association (AGIA), and the Lightning Ridge Miners Association (LRMA). Now, after three years of discussion, correspondence, and a plethora of drafted documents, and what seemed to be a never ending train of ideas and criticisms, a final draft nomenclature has been agreed-to, ratified, published, and is presented in this paper.

The Australian Gemstone Industry Council (AGIC) has accepted this nomenclature in its final draft, as has the GAA’s 1996 and 1997 Federal Conferences in Tasmania and Perth — albeit with one or two small amendments to the final draft. Now the AGIC hopes to actively progress production of a full colour publication and video on this opal nomenclature for distribution on a world-wide basis over the next twelve months. As Chairman of the GAA’s Opal Nomenclature sub-committee I would like to express my gratitude to Jack Townsend (South Australia), Kathy Endor (Queensland) and Andrew Cody (Victoria) for their untiring efforts and fruitful discussions. Also, this author wishes to express his appreciation for the work and constant liaison of the AGIA sub-committee members Glenn McKean, Drago Panich, Peter Sherman, and Peter Evans, as well as the generous support and hospitality offered by members of the LRMA — in particular Joe Schellnegger, Maxine O’Brien, and Frank Palmer.

I would encourage all members of the GAA to read and to use this nomenclature — in their every day activities, such as buying and selling, and in scientific correspondence and lectures. This nomenclature remains, according to GAA Past President Ronnie Bauer and the AGIA’s Andrew Cody, a ‘living document’. As time passes there will be, no doubt, more discussion and criticism of this nomenclature.This will be most welcome, as are any questions — all of which may be forwarded in writing to the GAA’s Opal Nomenclature Sub-committee either care of the Federal Office of the GAA at P.O. Box A791, Sydney South NSW 1235, or direct to the author at P.O. Box 692, Sutherland NSW 2232.

The nomenclature and classification of opal, that follows, is reproduced, verbatim, from the Resolutions of the Federal Council of the Gemmological Association of Australia (dated 17th May, 1997).

 

Opal Nomenclature and Classification

 

Introduction

Opal is Australia’s National Gemstone. Australia produces 95% of the world’s natural precious opal supply. This nomenclature encompasses all types and varieties of opal to provide a standardisation of terminology but does not establish any valuation methodology.

The Australian Gemstone Industry Council Inc., in collaboration with the Australian Gem Industry Association Ltd., the Gemmological Association of Australia Ltd., the Lightning Ridge Miners Association Ltd. And the Jewellers Association of Australia Ltd., has produced the following nomenclature for the classification of opal.

 

Opal Classification

Opal is a gemstone consisting of hydrated amorphous silica with the chemical formula SiO2.nH2O. There are two basic forms of opal described by visual appearance.

Precious Opal – is opal which exhibits the phenomenon known as play-of-colour, produced by the diffraction of white light through a micro-structure of orderly arrayed silica spheres to produce changing spectral hues.

Common Opal and Potch – is opal which does not exhibit a play-of-colour. The distinction between common opal and potch is based on formation and structure. Potch is structurally similar to precious opal but has a disorderly arrangement of silica spheres. Common opal shows some degree of micro crystallinity.

 

Types of Natural Opal

Natural opal is opal which has not been treated or enhanced in any way other than by cutting and polishing. There are three types of natural opal, with varieties described by the two characteristics of body tone and transparency.

Natural Opal Type 1 – is opal presented in one piece in its natural state apart from cutting or polishing and is of substantially homogenous chemical composition.

Natural Opal Type 2 – is opal presented in one piece where the opal is naturally attached to the host rock in which it was formed and the host rock is of a different chemical composition. This opal is commonly known as boulder opal.

Natural Opal Type 3 – is opal presented in one piece where the opal is intimately diffused as infillings of pores or holes or between grains of the host rock in which it was formed. This opal is commonly known as matrix opal.

 

Varieties of Natural Opal

The variety of natural opal is determined by the two characteristics of body tone and transparency.

 

Body Tone

The body tone of an opal is different to the play-of-colour displayed in precious opal. There are three varieties of natural opal based on body tone. Body tone refers to the relative darkness or lightness of the opal when ignoring the play-of-colour.

Black Opal – is the family of opal which shows a play-of-colour within or on a black body tone by reference to the AGIA Body Tone Chart N1, N2, N3 and N4 when viewed face up.

Dark Opal – is the family of opal which shows a play-of-colour within or on a dark body tone by reference to the AGIA Body Tone Chart N5, N6 when viewed face up.

Light Opal – is the family of opal which shows a play-of-colour within or on a light body tone by reference to the AGIA Body Tone chart N7, N8 or N9 when viewed face up. The N9 category is referred to as white opal.

Opal with a distinct coloured body (such as yellow, orange, red or brown) should be classified as black, dark or light opal by reference to the AGIA Body Tone Chart with a notation stating its colour hue.

Australian opal body tone chart indicator

Transparency

Opal shows all forms of diaphaneity and ranges from transparent to opaque. Natural precious opal which is transparent to semi-transparent is known as crystal opal. Crystal opal can have either a black, dark or light body colour tone. The term “crystal” in this context refers to appearance not a crystalline structure.

 

Opal Treatments

Opal can be subjected to various types of treatment. Present CIBJO guidelines state that any method of treatment other than standard cutting and polishing must be disclosed and the process used specified on all invoices, advertising and commercial documents. Types of treatments include colour enhancement, heating, painting, dying, resins and waxes, oiling or any application of chemicals. Opal is treated to change its natural appearance, structure or durability. Opal is colour enhanced in opal inlay jewellery where usually a thin solid crystal opal has black paint or glue applied or set above black painted jewellery.

 

Composite Natural Opal

Composite natural opal consists of natural opal laminates, manually cemented or attached to another material. The opal component is natural opal. There are three main forms of composite opal:

Doublet Opals – are a composition of two pieces where a slice of natural opal is cemented to a dark base material.

Triplet Opals – are a composition of three pieces where a thin slice of natural opal is cemented to a dark base material and a transparent top layer, usually of quartz or glass.

Mosaic and Chip Opals – are a composition of small flat or irregularly shaped pieces of natural opal cemented as a mosaic tile on a dark base material or encompassed in a resin.

 

Synthetic Opal

Synthetic Opal is material which has essentially the same chemical composition and physical structure as natural opal but has been made by laboratory or industrial process. Synthetic composites exist as synthetic doublets, triplets or mosaics and must be disclosed as synthetic composites.

 

Imitation Opal

Imitation Opal is material which imitates the play-of-colour of natural opal, but does not have the same physical and chemical structure or gemmological constants as natural opal.

 

Classification Reports

Classification reports for the following types of opal should include these details:

 

Natural Opal

  1. Type of opal
  2. Variety of opal as Black opal, Dark opal or Light opal with a body classification from N1 (Black) to N9 (White) based on the AGIA Body Tone Chart.
  3. Transparency as opaque, translucent or transparent. Note if it is crystal opal.
  4. Weight and dimensions

 

Treated Opal

  1. Type of opal
  2. Variety of opal as Black, Dark or Light opal
  3. Transparency as opaque, translucent or transparent. Note if it is crystal opal.
  4. Type of Treatment and process if known
  5. Weight and dimensions

 

Composite Opal

  1. Type of composite as doublet, triplet, mosaic or chip opal
  2. Treatment process, where relevant
  3. Dimensions

 

Synthetic and Imitation

  1. Gemmological category including manufacturer (if known)
  2. Description (Body Tone)
  3. If composite, mention type as doublet, triplet, mosaic or chip
  4. Weight and dimensions, only dimensions if composite

 

Origin

Any indication of the origin of opal by the use of geographical location should not be used unless it is qualified as an indication of the type of locality only as recommended by the International Confederation of Jewellery, Silverware, Diamonds, Pearls and Stones (CIBJO) such as Lightning Ridge type black opal.

 

How to Use the New Opal Nomenclature

This nomenclature for opal has been designed for use throughout the gemstone and jewellery industry, not only in Australia but internationally. While preparing this nomenclature, the sub-committee has been cognisant of conventions of international trade organisations, such as the International Confederation of Jewellery, Silverware, diamonds, pearls and stones (CIBJO), the International Colored Gemstone Association (ICA), as well as the linguistic problems associated with different languages and the differing connotations these languages may place on an internationally acceptable nomenclature.

This new nomenclature has not been designed to force any changes to the various colloquial terms used to describe opal in Australia, or indeed in countries overseas such as Mexico. Colourful language, Australian colloquial terms for opal, and terms that have been a part of the Australian scene for hundreds of years have added significantly to the mystique and folklore of everyday language used on the opal mining fields. Expressive local terms and older historical terms always will exist in the opal miner’s vocabulary. These will remain to have their rightful place in our gemstone history and in the tale-telling for years to come.

The purpose of the nomenclature, therefore, remains to provide a basic description of the gemstone we all prize and know as opal. This nomenclature is for everyone to use and understand. Simple descriptive terms, that can be used by the majority of people, from the customer to the scientist, have been chosen. These provide the gemstone industry as a whole with a logical and unbiased way of grading and evaluating opal. However, simple terms do become difficult when the many different types, formations, pseudomorphic fossil replacements, mineralogical types, and geological occurrences of Australian opal are considered.

Having said that, there are a few items of terminology which it is hoped this nomenclature will remove from common usage. In particular, the terms that have been deliberately removed, due to the linguistic problems they create, are ‘semi-black’, ‘grey’, and ‘solid’.

To begin with the first part of the nomenclature, mention is made of precious opal, potch and common opal. The best way of determining the difference between these is to observe whether or not the opal you are viewing shows the phenomenon which we all know as play-of-colour. It is possession of this optical phenomenon for which opal is most prized. The differentiation between these basic forms of opal is therefore quite simple. If the opal displays a play-of-colour it is termed precious opal. If a play-of-colour is not displayed, then the opal is either common or potch opal. While it is recognised that the term precious is neither a scientific nor gemmological term, it is retained in this nomenclature for simplicity, and with the intention of further enhancing the value of opal as a gemstone by removing it from any historical association with ‘semi-precious’ gemstones.

In an attempt at keeping the nomenclature simple to use, the terms common opal and potch opal have not been separated. It must be recognised, however, that there are distinct mineralogical differences between potch and common opal. (Jones & Segnit, 1971).

The term ‘solid’ has been removed from opal terminology, for the simple reason that all types of opal are essentially solid from a scientific point of view. That is, opal does not exist naturally either as a liquid or a gas. ‘Solid’ has been replaced by the gemmological term natural opal. Correlating with this use is the recommendation that when describing doublets and triplets that the term composite be used instead of ‘assembled’. This also is the terminology currently recommended by CIBJO.

Essentially there are three types or forms of natural opal, which are termed simply opal, boulder opal and matrix opal. Perhaps the most contentious issue in the nomenclature concerned introduction of the term body tone, to describe the comparative lightness or darkness of an opal as distinct from its play-of-colour. Technically, it would have been best just to have two types of ‘body tone’ — either ‘black or white’ or just ‘light or dark’. However, the sub-committee rightly decided not to attempt to change too much of the terminology that had been in common use for over a hundred years. So, inclusion of the term black opal was considered to be an imperative. Following much discussion the term body tone was included in the nomenclature to describe the comparative lightness or darkness of opal — irrespective of its play-of-colour. The term tone, which is used by colour science, is in agreement with terminology used internationally to describe the lightness or darkness of particular hues or colours.

The Scale of Body Tone ranges from N1 to N9. The prefix “N” reflects the neutral tone of this scale.The steps in the scale of body tone, which are arranged to indicate approximately equal decreases of darkness, are difficult to reproduce accurately on the printed page. A rough gauge can be obtained by printing this scale with the assistance of a good computer and a quality laser or ink jet printer.

After examining current industry standards, the N4 category was decided to be the cut-off point for black opal. The AGIA is currently attempting to produce a scale of body tone, using commercially available computer scanning devices and suitable software. However, at the time of publishing this paper, this scale is not yet available. The current reference, used by the Lightning Ridge Miners Association, is the neutral tone scale specified in the American Geological Society’s Rock-colour chart † . This has proved to be a good guide, for in most instances it will be possible to correlate the different ‘tone scales’ into a simple and repeatable system. An acceptable descriptive term was sought also to describe those opals that have distinct body colours or hues, such as those displayed by both Mexican fire opal and honey opal from Lightning Ridge — considerable amounts of which consists of common or potch opal. However, as an acceptable all round term could not be found to describe these opals, the committee decided to describe them by determining their body tone/s, their primary and secondary body colour/s or hues, and their transparency.

To determine the body tone of an opal, then, one examines the piece of opal, face-up, and determines (by visual comparison) its position in the scale of body tone.

  • If the tone of the opal appears darker than N4, then the opal may be classified a black opal. Consequently, any opal with a body tone darker than N4, irrespective of hue, can correctly be termed black opal. Some boulder opal possesses this body tone, so it is very correctly termed black boulder opal. It is also appreciated that some very dark red Mexican-type opals would have dark enough body tones to be categorised as black opal.
  • If the opal is lighter than N4, and its tone corresponds to N5 or N6 on the scale of body tone, then it is classified as dark opal. If, in addition, this opal has a decided hue colour, it is additionally classified as, for example, a dark blue opal.
  • If, on the other hand, the tone of the opal corresponds to N7, or lighter, it is classified as light opal. If this light opal also has a hue, then it is termed, for example, a light yellow opal.

When to term an opal a crystal opal also provided considerable discussion. The key to classification as crystal opal is really the transparency of the opal. Perhaps a better term would have been ‘transparent opal’; but any change in terminology from crystal to ‘transparent’ may take many many years to progress. The obvious problem with the term crystal opal is, of course, the basic fact that that opal has no crystal structure. Again the sub-committee decided that it was unwise to change a term that had been in common use for so many years. The sub-committee further believes that overseas gemmological communities may yet force a change in this usage, if strict terminology is ever to be implemented.

The range of transparency considered acceptable for defining crystal opal (transparent to semi-transparent) was taken straight from Robert Webster’s discussion on transparency in his world-renowned textbook Gems. The committee decided that transparency did not need to be re-defined in the nomenclature; but just stated as a classifying category.

To grade the transparency of an opal with the nomenclature, how transparent the opal is must be determined. If the opal is only translucent, then it is not termed crystal opal. It should be remembered that in some instances the play-of-colour of crystal opal will be so strong or brilliant that assessment of transparency, by the normal ‘read-through’ criterion, may not be possible as the opal can not be ‘read-through’. When this occurs the best test of transparency would be to ‘look-through’ the opal with transmitted light. If transparency exists then this will be readily apparent. If the material remains only translucent, then it is correctly labelled as light opal. It is hoped that future scientific advances may yield a better and more accurate method of assessing transparency.

A note also should be made concerning the removal of the term ‘jelly’ opal. The basic facts are that due to the extreme transparency of this opal it becomes a type of lower quality crystal opal that displays subdued low quality play-of-colour. In spite of any restriction applied by this terminology the term ‘jelly’ opal will probably remain in colloquial use for many years to come.

The description of composite stones requires only a small change in nomenclature. Instead of these opals being described as ‘opal doublets’ or ‘opal triplets’, the nomenclature emphasises their composite nature by terming these doublet opals and triplet opal s. In this terminology, which emphasises the composite nature of these opals, it is the first word of the term that precisely identifies the material.

The rest of the nomenclature discusses opal treatments, synthetics and imitations. These are not associated with the descriptive nomenclature for natural opals, but have been included to complete the nomenclature. These descriptions are in accordance with the latest edition of CIBJO’s Classification of materials and Rules of application for diamonds, gemstones, and pearls.

 

Acknowledgements

The author wishes to thank Peter Sherman and Frank Palmer for providing specimens for examination. Rudy Weber’s photographic talents are gratefully acknowledged.

 

Sources

  • Australian Opal & Gem Industry Association
  • Australian Gemmologist (publication)

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“Aurora Australis” (picture and information courtesy of Altmann & Cherny)

The “Aurora Australis” was found in 1938 at Lightning Ridge and is considered the world’s most valuable black opal. The oval, cut and polished stone has a harlequin pattern with dominant red, green and blue colors against a black background. It weighs 180 cts. and is 3 inches by 1.8 inches. The rarity of the opal comes from its size and strong, vibrant colour play. It weighs 180 carats and its dimensions are 3 inches x 1.8 inches. Dug from an old sea-bed it has the distinctive impression of a star fish on its back. It was valued at AUD$1,000,000 in 2005.

The Aurora is the first large, fine, Australian opal mentioned in literature. Charlie Dunstan had found another large opal previously, but its blue-green colour play was not considered valuable at the time – although the stone weighed about 12 ounces (close to 1860 carats). At a depth of approximately six metres, Charlie found the brilliant gem. This treasure was rumored to have brought Dunstan 100 pounds. Altmann & Cherny purchased the opal in a semi-rough state (a rub).  They cut and polished the opal into its oval shape, and realising what a true gem they had, they named it the “Aurora Australis” after the bright southern lights in the night sky.

“Fire Queen”

In November 1906, Charlie Dunstan found “Dunstan’s Stone” (Later renamed to “The Fire Queen”), at the Angledool Diggings. She weighed in at about 6.5 oz. or nearly 900 carats. She was the largest nobby to date – alive with colour – “truly a marvelous gem, too beautiful for words!”

After selling the stone for a paltry £100 to an unknown buyer on a trip to Angledool, the story goes that Dunstan got drunk and “lost” two other big stones. In November 1910, Charlie Dunstan was found dead in his hut from a gunshot wound to the head. The verdict was that he had committed suicide.

After being originally sold for a mere £100, the stone changed hands several times, each new buyer finding it difficult to sell as there was hardly any market for big black opals in those days. By 1928, however, it was in the Chicago Museum, valued at £40,000 after being renamed “The Fire Queen”. In the 1940’s, it was then resold to J.D. Rockefeller for £75,000, who donated her to his prestigious family collection.

“The Black Prince”

‘The Black Prince’, originally known as ‘Harlequin Prince’, was found in 1915 at Phone Line by Urwin and Brown. Perhaps the least significant of the four notable stones from the same claim, this gem weighed 181 carats and displayed a flag pattern one side and the other was red. There was a sand hole in the face.

This black opal was acquired in England by a wealthy American serviceman, and later donated to the New York Museum of Natural History. Later ‘Prince’ became part of the collection at Forest Lawn Memorial Cemetery, Los Angeles, and was stolen at the same time as ‘Pride of Australia’. ‘Flamingo’ was the largest of the four notable stones in the Phone Line patch, weighing over a quarter of a pound at 800 carats! This black opal plus ‘Black Prince’, ‘Pride of Australia’ and ‘Empress’ were the ones for which Sherman paid Urwin and Brown £2000 in about 1920. This was the most ever paid to that date for four black opals. Ernie’s sister Bertha named the stones.

“Pride of Australia / Red Emperor”

(picture from www.resources.nsw.gov.au)

‘Pride of Australia’, also known as ‘Red Emperor’, was found in 1915 by Tom Urwin and Snowy Brown at Phone Line (off Fred Reece Way). The Pride of Australia is shaped like the continent. The 2″ x 3″ opal has black and blue veins interlaced with brilliant red streaks. By 1954, it had toured at least five World Fairs as “the greatest opal of Australia, and therefore the greatest opal in the world.”

This double-sided gem cut to a 225 carat stone that just fit into a tobacco tin. There were two distinct colour bars. The one on the back was much lighter and almost harlequin, totally different to the main bar of dark, rich flashes of colour. Ernie Sherman bought ‘Pride’ plus another three stones from the miners for £2000 around 1920. It was the highest price ever paid for four black opals. The ‘Pride of Australia’ was valued in 1931 at £2000 on its own, and was sold in the 1950s from the Percy Marks Collection, Sydney.

Dr. Eaton examines his “Pride of Australia” opal
with the Collector of Customs while guards look on.

In 1954, Dr. Hubert Eaton was the President and Founder of the world-famous Forest Lawn Memorial Park in Glendale, CA, and owner of one of the most important gem collections in the United States. Percy Marks, Ltd., located in Sydney, Australia, was home to the opal when Dr. Eaton was craving for his collection. This firm was supposed to have the greatest opal collection in the world. The Pride of Australia was sitting in the display window when Dr. Eaton arrived.

To Dr. Eaton’s dismay, the opal was not for sale. Dr. Eaton chose several other opals and told the firm that he wanted all of these opals plus the Pride — if the Pride wasn’t thrown in with the deal, then there would be no deal. Dr. Eaton wrote in a letter to his assistant, “Well, to cut a long story short, after quite a time, it ended by my going to the bank and getting a draft payable to Percy Marks, Ltd.”

Some say the price was £150,000, but Greg Sherman reckons not more than £50,000 was paid for this mostly green-shot-with-orange black opal. ‘Pride’ was later stolen from the new owner, Forest Lawn Memorial Cemetery, Los Angeles.

“Empress of Australia”

‘Empress of Australia’ was mined in 1915 from the same patch on Phone Line as ‘Pride of Australia’ by Urwin and Brown. First known as ‘Kaleidoscope Queen’, then ‘Tartan Queen’, this stone measured 3 x 2 3/4 x 2 1/4 inches in the rough.

The stone was shaped out and polished to reveal the glowing patches of red to best advantage, probably weighing 500 carats. Later, down at the pub, this, the most colourful black opal from the claim, slipped through the fingers of a local admirer, and fell to the floor, breaking into two pieces. Two almost matching stones were cut out of the first piece, each measuring 2 inches long and weighing 20 carats. Ernie Sherman’s daughter designed a beautiful pendant for one half. The second piece of ‘Empress’, measuring 1 3/4 x 1 1/2 inches and weighing 50-60 carats, was mounted in a gleaming necklet of brilliants.

“The Flame Queen”

The ‘Flame Queen’ was mined on Bald Hill in 1918, not far from where Dunstan mined ‘Queen of the Earth’ in 1906. Three partners, Jack Phillips, Walter Bradley and “Irish” Joe Hegarty took over a partially dug claim that was abandoned by a miner who left to fight in World War I.

Lightning Ridge was a risky place to speculate for opals. The early miners used picks and shovels, battling fatigue and hunger and desperate to find an opal-rich shaft. Hegarty completed the partially dug tunnel, but when he reached the opal level, the site appeared worthless.

The opal-rich clay, usually around 30 feet down the shaft, did not reveal any colour, which indicates the presence of gemstones. Once Hegarty reached the clay, he and Bradley tunneled vertically, a dangerous procedure that could result in the collapse of the entire site.

At this level, with little ventilation and light, Bradley discovered a “great nobby”. Close to 35 feet below the surface, in a tunnel little more that 2½ feet wide, he was hoisted up so that he could examine the stone under daylight.

The story goes that Walter Bradley took a “bite” at “a great black nobby” with his steel snips… and revealed the brilliance of opal within. They were offered £7 in the rough for the stone, which they refused. Of the three partners, Bradley was the most skilled lapidarist and had the best equipment to cut and polish the rough. It revealed a dazzling red domed center with a greenish blue border. The three men, broke and exhausted from their labor, hungry from scarce food supplies, hastily sold the opal to a gem buyer for 93 pounds.

Phillips, Bradley & Hegarty were the lucky miners, who shared the £93 that Ernie Sherman gave them for this collector’s piece. Cutting it would have spoilt the unique pattern. John Landers reported that the architecture “made this stone!” A black nobby as big as the palm of a hand, ‘Flame’ weighed 253 carats. An oval, 2 3/4 inches x 2 1/3 inches, with a dome that a half-crown would not cover, displayed a broad bronze-red flash. The ½-inch dome was framed with a high emerald green 3/8-inch band (then electric blue from another angle). Thus, the appearance of a ‘Poached Egg’, the rather unflattering nickname that was given to ‘Flame’.

One writer described the stone thus: “Suppose you put an egg in a frying pan. Directly the egg hits the pan, the white spreads out, leaving the yolk standing in the centre. This is what the exquisite stone looks like, only the yolk is a striking blood-red, raised above and surrounded by beautiful blue-green opal.” The cut and shape are highly unusual and enhance the natural formation of the stone. Under differing lights and angles, the stone reflects numerous combinations of color in a unique and remarkable way.

A Brisbane jeweller submitted the stone to the Queensland Geological Survey. It was established that traces of ginko, a fossil plant (Chinese maiden hair fern), occurring in Jurassic rocks but not in any opal deposits, were impressed on the back of the gem. The asking price for this unusual opal has continued to climb over the years with each change of hands. In 1925, an offer of £2000 was made. In 1948, the stone was valued at £5000. In 1973, $US 32,000 was paid. In 1980, ‘Flame’ was for sale again at a million dollars! As of 1992, the stone was back home in Australia. In 2003, ‘Flame’ was put up for auction at Christie’s in New York but was passed in for an undisclosed reserve. (Estimated at US$250,000) Current photos confirm the beauty of this gem and no sign of crazing after 86 years of to-ing and fro-ing.

“Olympic Australis”

(picture and information courtesy of Altmann & Cherny)

The “Olympic Australis” is reported to be the largest and most valuable gem opal ever found. It was found in 1956 at the famous “Eight Mile” opal field in Coober Pedy, South Australia. A miner working his claim found the opal at a depth of 30 feet. It was named in honor of the Olympic Games, which were being held in Melbourne at the time. This extraordinary opal consists of 99% gem opal with an even colour throughout the stone, and is one of the largest and most valuable opals ever found. The balance of 1% is the remaining soil still adhering to the stone. It weighs 17,000 carats (3450 grams) and is 11 inches long (280 mm), with a height of 4¾ inches (120 mm) and a width of 4½ inches (115 mm).  It was valued at AUD$2,500,000 in 2005. Due to the purity of the opal it is anticipated that upwards of 7000 carats could be cut from the piece.  However owing to it’s uniqueness, the opal will remain exactly as found.

Halley’s Comet

“Halley’s Comet”, is recorded in the Guinness Book of Records as the world’s largest uncut black opal nobby. The massive stone was found by a group of opal miners on the Leaning Tree Claim at Lightning Ridge known as “The Lunatic Hill Syndicate” about the time “Halleys Comet” appeared in Australian skies.

It weighs 1982.5 carats and measures 100 x 66 x 63 mm, or 4 x 2-5/8 x 2-1/2 in. Halley’s Comet was for sale in 2006 for AUD $1.2 million. The gem has a thick gem quality green and green/orange colour bar and is the largest gem nobby to be found at Lightning Ridge to date.

“Butterfly Stone / The Red Admiral”

The Red Admiral’ or ‘Butterfly Stone’ was discovered during World War I on the ‘Phone Line’ field. Reported to be 51 carats, the stone is of extraordinary beauty, with a predominant red pattern equally visible from all angles. It wasn’t until 1920 that the stone was given the name “Butterfly” because of its resemblance to the British butterfly, the Red Admiral.

Len Cram says of this stone, “If you turn this magnificent gem on its side it changes from a butterfly to a full-length picture of a Spanish dancer in traditional broad ruffled dress, perfect in pose and movement, aflame with fiery lights.”

It passed through a number of hands, including Percy Marks and a Queensland grazier, before being purchased by the late Mrs Drysdale of Sydney. As of 2004, it was back in the care of Percy Marks & Co.

Other Stones

Other fine named gem opals from Lightning Ridge include Red Flamingo (1914), Queen of Alexander (1918), Harlequin Flame (1919), Grawin Queen (1926), Sunset Queen (1928), Light of the World (1928), Pandora (1928), Queen of Australia (1931), and Rainbow Stone (1933).

Shrouded in mystery, the ‘Hope Opal’ rests in obscurity compared to its famous collection cousin, the Hope Diamond. Also called the ‘Aztec Sun God Opal’, the 35 carat transparent blue gem with play-of-colour, features a carved human face surrounded by sun rays. Assumed to be Mexican when catalogued in 1839, the opal’s origin remains unknown.

September 2003 saw the discovery of ‘The Virgin Rainbow’ (pictured above), a 63.3mm Black Crystal Opal Belemnite Fossil in a ‘pipe’ shape, featuring gem quality colour, weighing in at 72.65 carats. The specimen was discovered at Three Mile Fields in Coober Pedy, in South Australia by Johnny Dunstan.

Sources:
The Black Opal Advocate
“Australian Precious Opal”, Andrew Cody, 1991.
Lapidary Journal

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The home of Australian Boulder Opal

Queensland produces boulder opal, an unique type of opal which is found attached to a host rock, ironstone. Boulder opal is unique to Queensland, and occurs in deposits in weathered sedimentary Cretaceous rocks in the west of the state.

Much opal mining in Queensland is carried out in ‘open cut’ mining operations, which is vastly different to shaft mining.

The Queensland opal fields are within a belt of deeply weathered Cretaceous sedimentary rocks known as the Winton Formation, which extends in a north-westerly direction from the New South Wales border at Hungerford stretching west of Cunnamulla, Quilpie, Longreach and Winton to Kynuna, a distance of about 1000 km.

Queensland’s opal mining fields are located in the west and southwest of the State, and include:

Yowah field (the southernmost field centred on the small town of Yowah—includes Black Gate)

Koroit field (north-east of Yowah)

Toompine field (east and south-east of Toompine—includes Lushingtons, Coparella, Duck Creek, Sheep Station Creek and Emu Creek)

Quilpie field (west and north north-west of Quilpie—includes some of the more productive mines in recent times—Pinkilla, Bull Creek, Harlequin, and probably the most famous of all, the Hayricks.

Kyabra-Eromanga field (west and north-west of Eromanga)

Bulgroo field (north of Quilpie field in the Cheviot Range—includes the Bulgroo, or Germans—and Budgerigar to the north )

Yaraka field—includes the mines in the Macedon Range, such as Mount Tighe

Jundah field (west of Jundah over the Thompson River—includes Jundah and Opalville mines)

Opalton-Mayneside field (centred on the old abandoned township of Opalton, and to the south in the Horse Creek – Mount Vergemont area)

Kynuna field—the most northerly field, south of the township of Kynuna.

Boulder opal is widely distributed in rocks in these areas, in generally elongated or ellipsoidal siliceous ironstone concretions or boulders ranging in size from less than a few centimetres to more than 20 cm. Concretions up to 5 cm across, known as ‘nuts’, may host a kernel of solid opal or contain a network of thin veins of opal through the ironstone. This variety of opal is prevalent at Yowah where the concretions form distinct bands—the well known ‘Yowah-nuts’.

These opal fields lie within a 300 km-wide belt of deeply weathered Cretaceous sedimentary rocks known as the Winton Formation. This extends north-north-west from Hungerford on the New South Wales border, west of the townships of Cunnamulla, Quilpie, Longreach and Winton, to Kynuna, a distance of about 1000 km.

Boulder opal is widely distributed in rocks in these areas, in generally elongated or ellipsoidal ironstone concretions or boulders, from a few centimetres, to up to 3 m across. The boulders may be confined to one or more layers or randomly distributed through the weathered sandstone. Their composition ranges from sandstone types (a rim or crust of ferruginised sandstone surrounding a sandstone core) or ironstone types (composed almost entirely of iron oxides).

The opal occurs as a filling or lining between the concentric layers or in radial or random cracks in the ironstone, or as a kernel in smaller concretions or nuts. (as found at Yowah and Koroit fields, the famous ‘Yowah-nuts’). Matrix opal is where the opal occurs as a network of veins or infilling of voids or between grains of the host rock (ferruginous sandstone or ironstone). Rare seam or band opal is also found and is typically encased in ironstone.

Pipe opal occurs in pipe-like structures which may be up to several centimetres in diameter within the sandstone and these structures may be hollow or opal-filled. Wood opal is occasionally found replacing woody tissue material.

As opposed to other sedimentary precious opal, boulder opal is attached to the ironstone, and stones are usually cut with the natural ironstone backing intact. Solid opals may be cut from the ironstone material where the opal is of sufficient thickness. Boulder opals are fashioned to standard shapes and sizes but are also cut in freeform shapes to highlight their individual beauty and to avoid wastage. Magnificent picture stones are also cut but these are mainly of interest to collectors rather than for jewellery use.

Quilpie opal fields

Looking for colour? In these parts there’s plenty of it, to be found both in the stories relating to the early settlement of the region and also that ‘colour’ which is sought from beneath the surface of the bush earth. Quilpie is best known as an opal town. It is often referred to as the home of the ‘Boulder Opal’ as the area is the largest producer of this type of opal in the world.

Located 980 kilometres west of Brisbane and 208 kilometres west of Charleville, Quilpie is the commercial and social centre of the Quilpie Shire. The name of the town was derived from the Aboriginal word ‘Quilpeta’ meaning Stone Curlew.

Relatively young, Quilpie was declared an official town on 29 April 1917, in the same year the railway line from Charleville to Quilpie was completed. Quilpie is rich in grazing history beginning with the pioneering efforts of families such as the Costello’s, Tully’s and Duracks.

Quilpie is located on the banks of the Bullo River in the famous Channel Country of western Queensland. The shire is supported through primary industries such as sheep, cattle grazing, oil, gas and opal mining. Apart from these main industries Quilpie Shire houses a keen and talented artistic community.

Winton opal fields

Rich in history, Winton was originally known as Pelican Waterhole and was first settled in 1875. The town is best known as the place that Banjo Paterson wrote Waltzing Matilda in 1895, whilst at Dagworth Station outside Winton. The first performance of the ballad was reported to be at Winton’s North Gregory Hotel on the 6th of April of the same year. Winton is recognised as the ‘home’ of Australian bush poetry, hosting the annual Bronze Swagman Award, one of the country’s most prestigious literary awards.

Queensland and Northern Territory Aerial Service (Qantas), Australia’s national airline was formed in Winton in November, 1920 and its first board meeting was held in the Winton Club on 21st February 1921.

During the Great Shearers’ Strike in the 1890s, 500 shearers camped just south of Winton and the town was placed under marshal law. This was the beginning of the foundation of the Labor Party.

Winton is famous for its water supply which thrusts its way to the earth’s surface from three artesian bores, all around 1,200 metres deep emerging at a temperature of 83 degrees celsius. The water is sourced from the Great Artesian Basin which provides water for most of Australia’s outback.

Winton is in the centre of Matilda Country, a diverse region in which vast mitchell grass plains are broken by magnificent coloured gorges, ridges and jump-ups. Visitors to the region will be amazed by the vastness of the plains and the undulating nature of the landscape. There is a wide variety of animal and bird life in the area, generally best seen around dusk and dawn on minor roads and tracks.

Day trips from Winton take visitors to Opalton, one of the oldest opal fields in Queensland; Combo Waterhole, where the swaggie of ‘Waltzing Matilda’ fame reputedly met his fate; the vintage sandstone homestead of Old Cork Station; and Lark Quarry, where 93 million year-old fossils capture a dinosaur stampede.

Opalton opal fields

The Opalton Field, also called the Fermoy Field was one of the largest and most extensively worked opal deposits in Queensland. It is a good example of typical opal country in western Queensland and offers the visitor the opportunity to experience first hand the remoteness and harsh conditions endured by the opal miners. Mining activity on the field is mostly limited to small-scale hand mining but some larger operations using heavy machinery are present in the surrounding area.

The Opalton Designated Fossicking Land was established in 1995 under the Fossicking Act 1994 by the then Department of Mines and Energy, with the co-operation of the Winton Shire Council and the landholder to provide for tourist and recreational fossicking. The Opalton Field is located about 124 km by road south-southwest of Winton. Travelling from Winton take the Jundah Road (mostly unsealed) and travel 15 km, turn left and travel a further 109 km (unsealed road) past Weona Homestead to Opalton. Visitors are requested not to call at ‘Weona’.

The Opalton area attracts large numbers of visitors and is popular with tourists as a place to “speck” or”noodle” fragments of opal or ironstone matrix from the surface or from the spoil dumps of old workings. However, known areas of shallow ground, such as the old Brilliant Claim area, may offer the more serious fossicker a chance to dig and find that outstanding gem.

Yowah opal fields

The Yowah opal field which includes the nearby area known as Black Gate is the southern-most opal mining centre of western Queensland. It is popular with tourists and fossickers as it has easy access from main roads and has shops, fuel, telephone, caravan park, and a permanent bore water supply. A small local population increases significantly during the winter season. The Yowah Fossicking Area has been established by the Department of Mines and Energy with the co-operation of the Council of the Shire of Paroo and the landholder to provide for tourist and recreational fossicking.

Yowah is about 160km west of Cunnamulla. Travelling towards Thargomindah, turn off to the right about 18km west of Eulo onto the Yowah / Toompine road and travel 48km via Alroy homestead to the Yowah-Quilpie turnoff. Continue a further 23km to Yowah; this last 23 km is unsealed. From Quilpie, drive 110km through Toompine to the Eulo / Yowah turnoff. Turn left and follow this mainly unsealed road for about 56km to the Yowah turnoff, and then continue the further 23km as above.

A feature of the Yowah field is the occurrence of precious opal in siliceous ironstone nodules generally referred to as “Yowah Nuts”. These nuts range from about 5mm to 200mm across, have a spherical or ellipsoidal shape, and show alternate concentric rings or bands of light and dark brown siliceous ironstone. There is sometimes a kernel of precious opal which is the main source of the gem. The nuts are found in layers (150 to 600mm in thickness) at depths up to 20m in a ferruginous sandstone, and are commonly associated with mudstone fragments or clay pellets. The main layer is located near the contact between the sandstone and underlying mudstone / claystone, but scattered nodules, and in some cases a second band, may occur above. The lateral continuity of the nut bands is somewhat difficult to predict owing to the irregular bedding of the strata, as well as the lack of any detailed
mapping. In some shafts, the nut band was not encountered, but the sandstone at its contact with the mudstone was found to be more ferruginous and cemented by partial opalisation into a hard band, which also contained opal in the form of seams and pipes.

The eastern part of the Fossicking Area has always been popular with tourists as a place to “speck” or “noodle” fragments of opal or ironstone matrix from the surface or shallow depth. In this area the main nut band appears to have been exposed at the surface, so that a layer of loose rubble of broken ironstone nut fragments covers the surface to a depth of about 600mm. Spotting chips of opal or fragments of matrix while digging through this
material is relatively easy with a bit of practice. Fragments may also occur on the spoil heaps of shafts from the old mining activities.

Opals from Queensland

Here’s a sample of the latest opals from Queensland currently available in our Australian opal catalogue:

Sources :

Queensland Holidays

“Opal in South Australia”, Mines & Energy Resources, SA

“Opal”, Qld Dept. of Mines & Energy

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FAQ Where are the opal fields in Australia? Where are opals mined? Where is opal mined? Where is opal mining done? When was opal discovered?

Although there are lots of opal mining towns in Australia there are four which have become household names – Coober Pedy, Andamooka, White Cliffs and Lightning Ridge. They are wild and unruly places surrounded by a moonscape of mullock humps where people fight against horrendous climate conditions in their search for precious gemstones. They are, as one observer noted, ‘monuments to the tenacious optimism of all mankind’.

(Opal mines are indicated by an orange dot)

Currently, Australia produces around 95% of the world’s opal for use in the jewellery industry. Other countries in which opal is found in small amounts include Honduras, Mexico, former Czechoslovakia, and Brazil, however these types of opal often differ in appearance. Australian opal is considered the finest in the world.

The Australian export market for opals in 1998-99 was estimated at $60 million compared with $69 million in 1997-98 and $85 million in 1996-97. Between 2000 and 2005, production figures for uncut gems varied between $100 million and $200 million.

Australia’s Opal fields lie in the three states of Queensland, New South Wales, and South Australia, along the site of the ancient ‘Great Inland Sea’, or ‘Great Artesian Basin’. White, or ‘Milky’ opal, is found in South Australia, Black opal is found in Lightning Ridge, NSW, and Boulder opal is found in Queensland. The best time to visit the opal fields is April to September. Summer should be avoided due to the high temperatures and possible heavy rains making road access impossible in some areas.

New South Wales (NSW) – The Home of Black Opal

New South Wales produces the largest proportion of Australian opal in terms of value. Lightning Ridge is famous for producing black opal, the darkest and most valuable form of opal. Read more >

 

Lightning Ridge, New South Wales

The Lightning Ridge opal mining fields are synonymous with world famous gem quality black opals. Unlike ordinary opals the black opal has carbon and iron oxide trace elements in it, producing the most sought-after opal in the world.

 

White Cliffs, New South Wales

White Cliffs, situated in north western New South Wales, produces predominantly white or crystal opal in the form of ‘seam opal’ (i.e. opal that forms in horizontal seams in the ground as opposed to small nuggets, or ‘nobbies’.) White Cliffs is also notable for producing ‘opal pineapples’, a strangely shaped opal fossil in the shape of a mineral crystal. These rare fossils are formed when a mineral crystal of glauberite (or ikalite) is first replaced by calcite and then opalised.

 

Queensland (QLD) – Boulder Opal Country

Queensland produces boulder opal, an unique type of opal which is found attached to a host rock, ironstone. Boulder opal is unique to Queensland, and occurs in deposits in weathered sedimentary Cretaceous rocks in the west of the state. Read more >

 

Quilpie, Queensland

Looking for colour? In these parts there’s plenty of it, to be found both in the stories relating to the early settlement of the region and also that ‘colour’ which is sought from beneath the surface of the bush earth. Quilpie is best known as an opal town. It is often referred to as the home of the ‘Boulder Opal’ as the area is the largest producer of this type of opal in the world.

 

Winton, Queensland

Winton is in the centre of Matilda Country, a diverse region in which vast mitchell grass plains are broken by magnificent coloured gorges, ridges and jump-ups. Visitors to the region will be amazed by the vastness of the plains and the undulating nature of the landscape. There is a wide variety of animal and bird life in the area, generally best seen around dusk and dawn on minor roads and tracks.

 

Opalton, Queensland

The Opalton Field, also called the Fermoy Field was one of the largest and most extensively worked opal deposits in Queensland. It is a good example of typical opal country in western Queensland and offers the visitor the opportunity to experience first hand the remoteness and harsh conditions endured by the opal miners.

 

Yowah, Queensland

The Yowah opal field is the southern-most opal mining centre of western Queensland. A feature of the Yowah field is the occurrence of precious opal in siliceous ironstone nodules generally referred to as “Yowah Nuts”.

 

South Australia – The White Opal Fields

South Australia has four active opal mining fields, Andamooka, Coober Pedy, Lambina and Mintabie. South Australia is largest producer of opal in terms of volume, and produces the white opal or ‘milky’ type of opal. Read more >

 

Coober Pedy, South Australia

Coober Pedy produces the bulk of the world’s white opal. The opal mining fields of Coober Pedy lie in the outback of South Australia, Stuart Range, 750 km north of Adelaide. Many of the locals in Coober Pedy prefer to live underground in dugouts where it is cool in summer and warm in winter.

 

Andamooka, South Australia

Situated 520 km north of Adelaide, Andamooka miners work over an area of about 52 square kilometres on the Arcoona plateau in shafts, large bore-holes, open-cut excavations and small tunnels.

 

Mintabie, South Australia

A well sinker, named Larry O’Toole, is credited with finding the first opal in Mintabie in the 1920’s. Mintabie is the newest opal field, discovered in the 1920’s, but was not aggressively worked until 1978, when good opal began to be found. Mintabie is situated 180km south of the Northern Territory border, and approx. 300km north of Coober Pedy.

 

Lambina, South Australia

Old miners claim that opal was first discovered at Lambina during the depression years of the early 1930’s. A minor rush in the late 1980’s occurred following discoveries by some miners at Seven Waterholes diggings.

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How to Cut and Polish Opal

FAQ:  How is opal cutting done? How do I cut rough opal? Where can I learn about cutting opals? How is opal polishing done? How can I polish an opal?

Opal cutting and polishing is a very specialised skill. Rough opal is normally purchased from the opal miners as ‘parcels’ (Bulk quantities of opal in its rough state). Potential buyers sort through the parcels and try to predict the value of stones which can be produced from the rough material.

However there is never any guarantee, as opal cutting can produce very unpredictable outcomes. Once the opal cutter has sorted through the parcel and decided which pieces are worth cutting, a diamond saw is used to cut the rough opal into ‘ rubs’ (opal in the rough shape of a stone). During this process, any excess material, cracks and potch (colourless opal) is cut off, and the piece of opal is cut into a basic stone shape.

opal cutting

Probably the most basic concept which any opal cutter needs is to keep the stone as large as possible, i.e. minimising waste and maximising the end size of the stone.

Each moment of cutting reduces the size of the stone, so control must be exercised.  The second basic concept is that opal can be ‘burned’ or may even crack if subjected to extreme temperatures. For this reason, water must always be used when cutting opal to avoid overheating due to friction.  ‘Burning’ a stone during polishing results in small pits forming on the surface thereby ruining the smooth surface and polish.

                   

After the stone has been cut on the saw by hand, the opal cutter will then normally place the stones on ‘dop sticks’, consisting of nails or lengths of wood dowling, using heated wax to adhere the stone to the end of the stick. This allows a greater degree of control of the stone on the cutting wheel, especially when the stone is small. The wax is softened on a burner to permit the fixing of the stone, which is first adhered with the face of the stone pointing upwards. The face of the stone is decided by the opal cutter, considering which side has the best colour, and the best shape for the stone.

The opal cutter then uses a series of diamond grinding wheels (coarse to fine) to shape and perfect the stone. Importance is placed on removing imperfections, such as sand spots, and removing saw marks and rough spots from previous stages. The face of the stone is shaped into a cabochon (dome shape) and the shape is decided depending on the stone (normally oval). Again, maximising the size of the stone is an important consideration.

The final stage for the face is polishing. Serium Oxide is used as a polishing agent on a felt wheel with water to give the stone a beautiful polish. If the cutter is happy with the shape, and the absence of scratches, grinding marks or imperfections, he removes the stone and sticks it back on the wax with the back facing up.

The back of the stone is cut on the same set of grinding wheels, this time producing a flat bottom for the stone, and an edge which tapers up to the ‘girdle’. The shaping of the girdle is an important and difficult part of cutting, and refers to the point on the side of the stone where the two top & bottom edges meet. This edge is used by jewellers to set the stone underneath the gold, to provide a secure setting.

Boulder opal can be significantly more difficult to work. The opal forms in tiny cavities in the ironstone, therefore the seams of opal that run through the boulder can be of very excellent quality but are very thin veins from .25mm to 20 mm thick. The ironstone is generally left as backing to support the stone.

Occasionally a thick vein is deposited allowing the cutter to cut the opal in cabochon, however frequently the veins are thin and wavy, so the cutter is challenged to cut and polish the piece following the deposition of the opal, resulting in an undulating or baroque surface. Stones are generally cut into freeform shapes, which is dictated by the opal deposition and flaws within the piece.

Ironstone is also significantly harder than opal, (opal is only as hard as glass) which provides another challenge for the opal cutter. Opal will grind much quicker than ironstone, so extra care must be taken when polishing a surface comprised of both materials.

So, now you know how to cut opal! Well, not exactly… this is only a very rough guide, and it takes a lot of practice to cut opal correctly. We recommend that learners get hold of some cheaper rough material to begin with, and to get a feel for the stone. Cutting and polishing opal is a great skill, and it’s also very rewarding to uncover such beautiful colour!

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FAQ: Where can I find an opal engagement ring? Are opals suitable for engagement rings? Where can I find opal wedding rings?

Australian opals, also known as “The Queen of Gems” are one of the most beautiful gemstones in the world. Opals display all the colours of the rainbow in an amazing moving diffracted colour pattern known as the play of colour.

This makes opals very unique and opal engagement rings are becoming increasingly popular amongst people looking for a different and original engagement ring a little different to the standard diamond engagement ring. Opals, unlike diamonds, are genuinely rare – as fast as opals are mined, they are sold. Diamonds are in fact very common, yet prices are kept high by vast stockpiling and strict supply control by monopolistic forces.

Unlike diamonds however (which are incredibly hard), opals are a relatively soft gemstone. Opals rank at about 5 or 6 on Mohs scale of mineral hardness, whereas diamonds rank at 10. To put this into perspective, opals are about the same hardness as glass. This is the most important thing to be aware of when considering an opal for an engagement ring. It is possible by mistreating an opal or treating it roughly, to damage or break the stone. This does not mean opals will break at the drop of a hat, it just means you need to treat them carefully to ensure you enjoy your opal for a lifetime!

When our customers tell us they are looking for an opal engagement ring, we always ensure they are aware of this factor before they start their search. There are a number of ways you can ensure the maximum security and durability of your opal, and you should consult the advice of experts before going ahead and buying an opal ring.

Opals Down Under has made hundreds of customised opal rings, and we’ve got engagement rings down to a fine art. Here are our tips to help you get the best out of your opal ring:

Tips for buying an opal engagement ring

  • Pick a rub-over (bezel) setting. This is pretty much a “must” for opal engagement rings, as rub-over settings provide much better protection and security for your opal. A thin bezel of gold follows and covers the edge of the stone, protecting damage from impact, and ensuring the stone stays securely in place. Claw settings are much less secure, provide little protection, and can wear down over time – especially in rings.
  • Boulder opal is harder wearing. Due to its very hard natural ironstone backing, Queensland boulder opal is more robust and has an advantage over other types of opal. For an opal engagement ring, boulder opal is ideal. Black opals, crystal opals, and white opals are also suitable, but do not have the same hard-wearing quality as boulder opals. Due to their unusual ‘free’ shape, boulder opals also lend themselves to more creativity in design.
  • Select a stone with a low cabochon (i.e. dome on top). Opals with a high cabochon are more exposed and vulnerable to impact damage, so if your stone has a flat or low cabochon top, it’s less likely to be damaged.

In summary, opals can make a beautiful lasting engagement ring if you make the right choice and treat the opal with respect. We have made many opal engagement rings for our clients over the years – simply email us your ideas and we’ll promptly draw up some designs for you.

The cost of your opal ring depends on the type of stone and setting you choose, as well as the size of your finger. Please don’t hesitate to contact us if you’d like to talk about having an opal engagement ring custom designed, we’d be more than happy to help. In the meantime, feel free to learn more about opals, or check out our current stock of opal rings.

 

 

 

 

 

 

 

 

 

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