If you have seen our online opal catalogue, you will have noticed a number of categories used to describe and classify each stone. This system of offering detailed information about each stone is based on the Australian Opal Ebusiness Association’s (AOEA) Opal Classification Standard, and aims to offer as much information about each stone as possible, allowing you to make an informed decision about your opal.

Clearly there is no substitute for viewing a stone in person, however we believe that our videos and pictures (in which we take meticulous care to represent our stones accurately), combined with this classification system, allows our customers to buy with confidence. Our clients are almost always pleased with their stone once it arrives, since as a general rule a stone is better in real life than in any photograph. For a good detailed run-down on how the value of an opal is determined, please see our article on valuing opals.

 

Our Classifications Explained:

ID – This is the unique identification number issued to each opal when it is processed.

Category – The preface Solid means that the stone is a natural cut & polished opal which does not have any kind of backing adhered to the stone to enhance the colour (as is the case with the partially fabricated stones – doublets or triplets). Queensland Boulder Opals, even though they have a natural brown ironstone backing which makes the stone darker, are still known as solid opals since this is the natural formation of the stone. Read more about the types of stones.

Black Opal refers to opal which has a dark grey to black body tone, and is generally mined in the Lightning Ridge area of New South Wales. As a general rule, black opal is the most valuable form of opal, since its dark body tone causes the colours to be more vibrant.

Boulder Opal is opal mined in Western Queensland which normally has a natural brown ironstone backing attached to the stone. Boulder opal usually has a very dark body tone and is thus generally the second most valuable form of opal.

Crystal Opal means any kind of opal which has a translucent or transparent quality (i.e. you can see through it). Translucent or transparent stones often have an enhanced clarity of colour, and for this reason it usually increases the value of a stone. The term Crystal Opal normally denotes opal with a very light body tone, however Black Crystal Opal refers to a crystal opal which has a dark body tone.

Semi-Black Opal refers to opal which has a light to medium grey body tone and is therefore not quite dark enough to be called black opal. These opals usually fall within the ‘dark opal’ category in the Body Tone Index. (See diagram further down) Semi black opal is generally found in Lightning Ridge, but is also found in White Cliffs and occasionally South Australia. This can be one of the lesser valuable forms of opal.

White Opal means opal with a white to light body tone, and is also known as milky opal. White opal is found in large quantities in South Australia, and the bulk of it does not have the same vibrancy of colour as found in other forms of opal. For this reason, it is generally one of the least valuable forms of opals. (High quality white opal is available however.)

Setting – In the case of jewelry, this indicates the carat of the gold used, and whether it is White Gold or Yellow Gold.

Weight – This refers to the carat weight of the stone. Five carats equals approximately one gram. The value of an opal is usually determined by calculate a price ‘per carat’ according to the colour and appearance of the stone, and this value is multiplied by the carat weight. When there are multiple stones, the carat weight of all stones combined is given.

Origin – The place in Australia where the stone was mined. See our article on Australia’s opal mining fields for more detailed information on each field.

Dimensions – The dimensions (width and length of the stone facing upwards) measured in millimetres. One inch equals 25.4 millimetres. In the case of a freeshape stone, the measurements are generally given at the widest points of the stone. When there are multiple stones, the dimensions of the largest stone are given.

Thickness – The measurement in millimetres of the stone’s thickness (i.e. looking at the side of the stone). This measurement is taken as close to the centre of the stone as possible. This includes any potch (colourless opal) or ironstone which is naturally attached to the back of the stone. In the case of boulder opal, the actual layer of opal can be less than 1mm thick. Our stones are cut with enough backing on them to support and stabilise the stone and give them a good shape. We never leave extra weight or thickness on the back of a stone to boost its carat weight or value.

Body Tone Index – (See Figure 1, below). This is a device used to classify the darkness of a stone. Generally a darker stone leads to more vibrancy of colour, however it depends on the individual opal. Boulder opals are always listed as having a body tone index of 0 since they cannot be classed in the same system as other opals due to their ironstone backing. Boulder opals generally have a very dark body tone however.

Transparency – Refers to the ‘diaphaneity’ (transparency) of an opal. Opaque means the stone is not transparent. Translucent means the opal has a semi-transparent nature. Transparent means you can see through the stone. This category is used to determine whether a stone has any of the properties of a crystal opal.

Shape – Refers to the shape of the stone. Freeshape means anything which is not in a standard oval shape. Cabochon refers to the dome on the top of the stone. A cabochon can effect the appearance and pattern of a stone – for example, crystal opals often look better with high cabochon, whereas black opals can look better with a low to medium cabochon. This is up to the individual stone however and relies on the skill of the opal cutter to maximise its beauty and pattern. Low Cabochon means it has a flat or hardly any dome. Medium Cabochon means it has a medium dome. High Cabochon means it has a high dome on the surface.

Colours – Lists each colour of the spectrum which is visible in the opal. Generally the most prominent colour is listed first, then the second most prominent colour and so forth. The rarity of colours is as follows – in order of the rarest (most valuable) to the most common (least valuable). Red; Orange; Yellow; Green; Blue. Red is therefore the rarest and most highly sought-after colour in an opal, and therefore fetches the highest price. Unusual colours may also occur, such as purple and aqua which can also enhance the beauty and value of a stone. Read more about how opal displays colours in our opal colour article.

Brightness – This is one of the most important ways in which we determine the value of an opal. There are three brightness ratings – Subdued, Bright, and Brilliant. These categories are quite broad and are intended to give a general indication of a stone’s brilliance. Subdued means the stone falls into this category with the least brightness – (note that it still may be a beautiful stone). Bright means the stone has a good level of brilliance and falls into the middle category. Brilliant means the stone is a real eye-catcher – has excellent brightness and falls into the top category in this classification. Brilliant is obviously the most sought-after and valuable property for a stone as it is a very desirable quality.

Pattern – This is a description of the arrangement of the colours on the face of the stone and how they appear to the eye. The most common pattern is Floral, which we use as a very broad description meaning a random and relatively indistinct pattern of opal colours. Most opals fall into this category.

More valuable patterns include Pinfire (small dots of colour sparkling like stars), Broad Flash (large sections of colour which flash brightly at certain angles), and Rolling Flash (a large section of colour in which a bright flash rolls across a section of the stone as you move it).

Even more valuable patterns include; Ribbon (Almost indescribable – Multiple rolling flashes which line up in different sections moving next to each other and in succession), Flagstone (large distinct blocks of colour), Straw (small and thin multiple lines of colour next to each other), Chinese Writing (thin strokes of colour which look like chinese writing).

The most valuable and rare pattern is Harlequin, in which blocks of colour lie next to each other and are of approximately the same size and shape (like a checkerboard). This pattern is extremely rare and is the legendary in opal circles. Many websites on the internet use the term Harlequin very liberally, so be wary of what you are buying. Traditionally a true Harlequin opal is extremely valuable and rare, and can fetch hundreds of thousands of dollars. In two decades of experience, our opal cutter has only ever seen two true Harlequin opals, just to give you an idea of their rarity.

Notes – This is where any extra description, special characteristics, faults or interesting attributes are mentioned.

Any questions? Don’t hesitate to email us!

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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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The Geology of Opal

FAQ :  How is opal formed? / How are opals formed? What is the Great Artesian Basin? What is potch? What causes the formation of opal?

A Simple Explanation

Opal is formed from a solution of silicon dioxide and water. As water runs down through the earth, it picks up silica from sandstone, and carries this silica-rich solution into cracks and voids , caused by natural faults or decomposing fossils. As the water evaporates, it leaves behind a silica deposit. This cycle repeats over very long periods of time, and eventually opal is formed.

 

A Detailed Explanation

Occasionally, when conditions are ideal, spheres of silica, contained in silica-rich solutions in the earth form and settle under gravity in a void to form layers of silica spheres. The solution is believed to have a rate of deposition of approximately one centimetre thickness in five million years at a depth of forty metres. If the process allows spheres to reach uniform size, then precious opal commences to form. For precious opal the sphere size ranges from approximately 150 to 400 nanometres producing a play of colour by diffraction in the visible light range of 400 to 700 nanometres.

Each local opal field or occurrence must have contained voids or porosity of some sort to provide a site for opal deposition. In volcanic rocks and adjacent environments the opal appears to fill only vughs and cracks whereas in sedimentary rocks there are a variety of voids created by the weathering process. Leaching of carbonate from boulders, nodules, many different fossils, along with the existing cracks, open centres of ironstone nodules and horizontal seams provide a myriad of moulds ready for the deposition of secondary minerals such as opal.

Much of the opal deposition is not precious. It is called “potch” by the miners, or common opal by the mineralogist, as it does not show a play of colour. Opaline silica not only fills the larger voids mentioned but also may fill the pore space in silt and sand size sediments cementing the grains together forming unique deposits, known as matrix, opalised sandstone or “concrete” which is a more conglomeratic unit near the base of early Cretaceous sediments.

The many variations in the types of opal depends on a number of factors. In particular, the climate provides alternating wet and dry periods, creating a rising or more importantly a falling water table which concentrates any silica in solution. The silica itself is formed either by volcanic origin or by deep weathering of Cretaceous clay sediments producing both silica and white kaolin often seen associated with the Australian opal fields. Special conditions must also prevail to slow down a falling water table in order to provide the unique situation for the production of its own variety of opal.

The chemical conditions responsible for producing opal are still being researched, however some maintain that there must be acidic conditions at some stage during the process to form silica spheres, possibly created by microbes.

While volcanic-hosted and other types of precious opal are found in Australia, virtually all economic production comes from sediment-hosted deposits associated with the Great Australian Basin. Australia has three major varieties of natural sediment-hosted precious opal – black opals from Lightning Ridge in New South Wales, white opals from South Australia, and Queensland boulder and matrix opal.

The formation of Boulder Opal

 

 Opal geology diagram

The Boulder opal found in Queensland forms in a slightly different method to other types of opal, forming inside an ironstone concretion. The concretion was formed due to ionisation, from sedimentary deposition. By definition, they are ionised concretions of varying hardness with an approximate opal composition of SiO2at 28%, Fe2O3 + AL203 at 68% and H2O at 1% composition.

The opal forms 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.

 

Facts about the Great Artesian Basin:

  • Is one of the largest freshwater basins in the world
  • Contains approximately 8,700 million megalitres of water
  • Underlies 22% of Australia
  • Covers a total area of 1.7 million km squared
  • Supports a population of 200,000
  • Underpins $3.5 billion of production annually

 

Sources:

“Opal in South Australia”, Mines & Energy Resources, SA
“Opal”, Qld Dept. of Mines & Energy
The Australian Gemmologist, Vol 21, #1, 2001. “Geology of Australian Opal Deposits”, L.J. Townsend.
“Lightning Ridge, Walgett & District”, information booklet, p13.
Queensland Boulder Opal Association

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FAQ: Are opals suitable for mens’ rings? Do men wear opal rings? Are opals strong enough for a man’s opal ring?

Opals are one of the world’s most amazing gemstones, and men all over the world are taking advantage of opal’s striking colours to create a unique, masculine piece of opal jewelry. However, it’s important to do your research if you’re thinking about buying a mens’ opal ring or having a custom opal ring made. Since men are usually much harder on their jewellery, mens’ rings usually need to be much harder wearing than ladies’ rings.

Since opal is not as hard as diamonds (see caring for opal for more details), you do need to take precautions to ensure your opal doesn’t get damaged. Opals rank at about 6 on the Mohs scale of mineral hardness, whereas diamonds rank at 10. To put this into perspective, opals are about the same hardness as glass. There are a number of ways you can ensure the maximum security and durability of your opal. Much like opal engagement rings, 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 mens’ rings down to a fine art. Here are our tips to help you get the best out of your opal ring:

Tips for buying a man’s opal ring

  • Pick a rub-over (bezel) setting. This is pretty much a “must” for mens’ opal rings, as rub-over settings provide much better protection and security for your opal (See the above photo for an example). 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 a man’s opal 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.
  • 14k gold is harder than 18k gold, so you might like to consider having your ring made in 14k gold. (It’s also cheaper). When it comes to mens’ opal rings, the harder wearing the stone and setting, the better.

Due to the varying sizes of mens’ fingers, we rarely stock mens’ opal rings in-store, but rather prefer to have rings custom made to suit the client. 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 a man’s opal 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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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:  What causes the colours in opal? How does opal get its colour? What is potch? Where do opals get their colours? What is the structure of opals? Why does opal have colour?

The colour of an opal is a magnificent thing. Unlike any other gem, opals can display all the colours of the rainbow in an iridescent, moving pattern of red, green, blue, yellow, purple, aqua, pink, and any other colour you can imagine. The pattern and arrangement of the colour which is displayed in an opal can take on many beautiful forms, and the movement of colour across the face of a stone is known as the ‘play of colour’.

This captivating miracle of nature has been admired by people the world over for centuries, and highly sought after for use in jewelry, museums and collectors’ pieces. The opal is arguably the most beautiful of all gems – at the very least it is highly unique and a true treasure of the earth. The very idea that such magnificent colours have been hidden under the earth in darkness since ancient times, and pulled out of the ground to display their opalescence in the light of day, is truly staggering.

But what causes the colors in an opal? What is different about opal that makes the colours dance and play across the face and burn in every colour of the spectrum?

The answer, put simply, is the diffraction of light. Much like a prism, which can refract white light and produce a rainbow effect, opals diffract the white light which is coming from above, displaying those amazing opal colours. To understand how this happens, it’s time for a lesson on the microscopic structure of opals;

The Structure of Opals

Basically, opal is made up of water and silica (the main component in glass). A silica solution forms when silica from under the earth mixes with water. This solution fills voids or is trapped in layers under the earth, and opal begins to form. Learn more about how opal is formed.

Over a long period of time, the solution settles and the water evaporates, allowing the gradual formation of layer upon layer of microscopic silica spheres. The spheres are formed because particles of silica spontaneously adhere to other particles which form around it. These spheres of range in size from 1500 to 3500 angstroms (1 angstrom is 1 ten millionth of 1 millimetre).

Opal under a microscope

Because they are spherical, there are tiny gaps remaining between the spheres (much the same as when marbles are placed together in a container). In these gaps between the stacked spheres, a water and silica solution remains. The spheres in an opal are not only remarkably uniform in size but are packed, in gem quality opal, in a very regular array. It is these tiny spheres and gaps which hold the secret of the opal’s colour. See image, left – An electron-microscope photograph of of the ordered structure in precious opal, showing its light-diffracting spheres.

 

The Diffraction of Light in Opals

When white light waves enter the top of an opal, they refract and bounce around inside the opal, through all the microscopic spheres and the gaps between the spheres. As the light passes through the spheres and gaps, it diffracts (splits). Like a prism, the opal splits the white light into all the colours of the spectrum, and the light eventually bounces back out the top of the stone, at which point we get an eyeful of beautiful opal colours. The opal is the only known gemstone that is able to naturally diffract light in this way.

You may have noticed that some opals don’t have all the colours of the spectrum. Many opals can only display blue colouring, for example. This is because the diameter and spacing of the spheres controls the colour range of an opal. Getting back to our colour diffraction theory, the size and angle at which light is split determines the colour produced.

Small spheres produce opal of blue colour only (the most common), whereas larger spheres produce red (the rarest colour). When the spheres inside the opal are bigger (about 3500 angstroms diameter) the red or orange colours are produced. At the other end of the scale, at about 1500 angstroms in diameter, the blue end of the spectrum is diffracted. Between these sizes the rest of the colours of the rainbow occur.

Therefore the rarity of the colours (most common to least common) is as follows: blue, green, yellow, orange, and red. Opals which display red can also display all the other colours of the spectrum. Therefore the possible combinations of colours in an opal can be seen as: blue only, blue-green, blue-green-yellow, blue-green-yellow-orange, and finally the full spectrum of blue-green-yellow-orange-red. For this reason, the presence of red in an opal can greatly add to its value, since it is somewhat of a rarity. Opals can also contain aqua and purple as well as the other ‘non-primary’ colours which are produced when two primary colours are combined. (For example, the green and orange between the primary colours of blue, yellow, and red).

It can also be deduced that the light diffraction in the voids is greatest when the sphere size is greatest. Therefore, generally speaking, red is usually the brightest opal color and blue is duller. 

Potch, also known as common opal, is any type of opal which does not display any color. In this case, the silica spheres may be absent, too small, or too irregularly arranged to produce colour. (Opal which does display colour is known as precious opal.) Potch is virtually worthless, although it often serves as an excellent dark backing for black opals which normally have a thin segment of precious opal naturally formed on a potch backing.

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FAQ :  What is the history of opal / opals? Who discovered opals? When was opal first found?

In a cave in Kenya, Louis Leakey, the famous anthropologist, uncovered the earliest known opal artifacts. Dating back to about 4000 B.C., they most likely came from Ethiopia. Historically, opal discoveries and mining progressed similarly to the ways diamond, emerald, ruby and sapphire were produced. As early humans found various gemstones, they slowly learned to work them into decorative shapes. As communities developed, gems became symbols of wealth.

Ancient painting depicting jewellery
In the Old World, Hungary mined opal for Europe and the Middle East, while Mexico, Peru, and Honduras supplied their own native empires with the gemstone. Conquistadors introduced New World opal to Spain when they returned with stones in the early sixteenth century.

Since the late 1800’s, Australia has dominated opal production with more than ninety per cent of the global output. Opal of differing qualities occurs in more than twenty other countries, including Zambia, Ethiopia, Guatemala, Poland, Peru, Canada, New Zealand, Indonesia, the USA, Brazil, and Mexico.

The modern name of the gem opal is derived from ancient sources: the Sanskrit Upala – which means “precious stone”; the Latin Opalus; and the Greek Opallios which both mean”to see a color change”.

Early races credited opal with magical qualities and traditionally, opal was said to aid its wearer in seeing limitless possibilities. It was believed to clarify by amplifying and mirroring feelings, buried emotions and desires. It was also thought to lessen inhibitions and promote spontaneity. The early Greeks believed the opal bestowed powers of foresight and prophecy upon its owner, while in Arabian folklore, it is said that the stone fell from heaven in flashes of lightning. To the Romans, it was considered to be a token of hope and purity.

Ancient Romans provided the first real market for opal. With a rich powerful empire, wealthy citizens acquired disposable income and a passion for gems. Opal, whose colours changed with every shift of light, was rarer than pearls and diamonds and destined to be the stuff of myths and dreams.

Mark Antony loved opal. Indeed, it is said that he so coveted an opal owned by Roman Senator Nonius that Mark Antony banished the Senator after he refused to sell the almond sized stone, reputed to be worth 2,000,000 sesterces. (US $80,000) Mark Antony is said to have coveted the opal for his lover, Cleopatra. Legend states that one Roman Emperor offered to trade one-third of his vast kingdom for a single Opal.

Writing before his death in 79 A.D., the Roman Pliny wrote of the opal as “Having a refulgent fire of the carbuncle (ruby or garnet), the glorious purple of amethyst, the sea green of emerald, and all those colours glittering together mixed in an incredible way.”

Pliny thought the opals came from India, but the gems so eagerly sought by Rome probably came from open cut mines in Hungary, situated near Cervenica or Cernowitz (now Czechoslovakia). He had been deceived by dealers who had probably hoped to capitalise on the appeal of “oriental” imports. Hungarian opals have a milk-white background, usually with a pin-fire, small-size colour display. During the Middle Ages, more than three hundred men worked the mines in Hungary. The mines in Eastern Europe were the only source of European opal until the Spaniards returned from the New World with Aztec opal.

In the Middle Ages, the opal was known as the “eye stone” due to a belief that it was vital to good eyesight. Blonde women were known to wear necklaces of opal in order to protect their hair from losing its color. Some cultures thought the effect of the opal on sight could render the wearer invisible. Opals were set in the Crown jewels of France and Napoleon presented his Empress Josephine a magnificent red opal containing brilliant red flashes called “The Burning of Troy.”

In the late 18th and early 19th centuries, opal began to fall out of favour in Europe. It was wrongly branded as bringing ‘bad luck’, and was associated with pestilence, famine and the fall of monarchs. Queen Victoria, however, did much to reverse the unfounded bad press. Queen Victoria became a lover of opal, kept a fine personal collection, and wore opals throughout her reign. Princess Mary, Duchess of Gloucester, gave an opal ring to her niece Queen Victoria in 1849. This opal ring had been previously owned by Queen Charlotte since about 1810.

Queen Victoria’s friends and her five daughters were presented with fine opals. Opal became highly sought after because the Royal Court of Britain was regarded as the model for fashion around the world and fine quality opal had recently been discovered in far-off Australia. In the latter years of Queen Victoria’s long reign, various Australian opal fields were discovered and worked.

Discovery in Australia

The first discovery of common opals in Australia was made near Angaston (SA) by the  German geologist Johannes Menge in 1849. Both the Queensland Boulder Opal and Lightning Ridge fields attracted miners in the 1880’s. Production of precious opal began at White Cliffs (NSW) in 1890, from Opalton (Qld) in 1896, and at Lightning Ridge (NSW) in 1905.

Before 1900, rough opal was sent from White Cliffs, the premier NSW opal field, to Germany to be cut and polished. Gradually, professional cutters began appearing on the fields. They rigged up old treadle sewing machines or bicycles, designing innovative cutting/polishing gear. In 1907 at Old Town, on the Wallangulla Opal Fields (later known as the Lightning Ridge Opal Fields), the first recorded cutter was Charles Deane. When the 3-Mile broke out in 1908, cutters worked at Nettleton on 3-Mile Flat. Lorenz had learned to cut in Germany. He used horizontal wheels with a hand crank and was an expert. He made doublets, jewellery, and was one of the first to buy opal by the carat. Many miners cut their own opal, and often very roughly.

Danger opal sign
A study of the many written accounts of the time suggests that most of those early Australian discoveries were accidental – a horse’s hoof kicked up opal-bearing rock, a boundary rider’s wife discovered a pretty pebble in a creek bed, a flock of sheep was struck by lightning during a rainstorm and the run-off from the storm uncovered opal at ‘Lightning Ridge’. A number of Queensland locations also came into their own during the Depression years, when men without work were willing to chance their luck.

When Australian opals appeared on the world market in the 1890’s, the Hungarian mines spread the idea that it was not genuine, probably because gems with such brilliant fire had not been seen before. By 1932, the Eastern European mines were unable to compete with the high quality stone being produced in Australia and ceased production, allowing Australia to assume the mantle of premier opal producer of the world, becoming famous for Lightning Ridge’s colourful and rare black and crystal stone.

In South Australia, Angaston was followed by Coober Pedy in about 1912, Andamooka in about 1930, and then Mintabie. During the depression of the 1930’s the industry declined until new finds in 1946 stimulated mining and, since then, there has been a spectacular increase in production. Now over 50% of world production comes from South Australia.

A History of Opal Mining in Queensland

The history of opal in Queensland is one of heartbreak, frustration, determination and at times success at incredible odds. Rich in myths and legends, Queensland is the birthplace of the Australian Opal Industry. Opal was first discovered in Queensland on Listowel Downs, south of Blackall in 1869. The first registered mine was in 1871 south of the present town of Quilpie. Among the early miners were Berkelman and Lambert, who worked a deposit on the Barcoo in 1872-1873, and whose opal attracted great interest at the Queensland Annexe of the London International Gem Exhibition in 1873.

Yowah opal fields
By 1875 there had been a number of wonderful finds and interest began to grow, but it wasn’t until, 1888 that Tullie Wollaston , a young surveyor turned entrepreneur from Adelaide made a determined effort to market the gem. In so doing he engraved his name forever across the annals of history. It was due to his sheer determination in convincing the gem merchants of the world to accept the gem that we now have a viable industry.

Opal gougers of last century were mostly shearers and station-hands who had little or no geological knowledge. George Cragg, a young stockman, discovered the northern opal fields on Warronbool Downs 100 kilometres south of Winton where the Opalton Field exists even to this day.

Two World Wars and droughts slowed the progress of Boulder Opal realising its full potential on the world stage. Although mining on a small scale continued it was relatively dormant. It was not until 1967, when Des Burton , a pharmacist from Quilpie become involved with Boulder Opal, unwittingly through his efforts, helped revitalise an industry. In the 1970’s he introduced modern opal cut mining techniques which revolutionised the opal mining industry.

Boulder Opal and the people that mine and deal with opal have supplied the industry a rich and colourful history, which has become part of Australia’s heritage. Opal has been discovered in Queensland from the Southern Borders of Western Queensland to as far north as Kynuna, this probably would be the largest opal field ever known, with opal mining centres in Winton and Quilpie.

Today the Queensland opal miner still exists, supplying the markets of the world with this most exquisite product, Queensland Boulder Opal.

Timeline – A History of Opal Mining in NSW

  • 1877 – Mining for precious opal in igneous rocks begins at Rocky Bridge Creek, a tributary of the Abercrombie River, in the Central West.
  • 1881 – Opal is discovered at Milparinka, near Tibooburra in the Far West.
  • 1884 – Opal is discovered in sedimentary rock at White Cliffs in the Far West.
  • 1889 – Precious opal is discovered at White Cliffs.
  • 1880s or 1891 – Opal is discovered in sedimentary rock at Lightning Ridge (Wallangulla) and other localities in the area, but its commercial value is not recognised.
  • 1890 – Precious opal mining begins at White Cliffs (continuing to 1915 then going into decline).
  • 1896 – Opal is discovered at Purnanga and Grenville-Bunker Field. These occurrences are near White Cliffs and so extend the size of that opal-bearing district.
  • 1897 – Opal is discovered in igneous rock at Tooraweenah, near Coonabarabran.
  • 1901 – Opal is discovered in igneous rock at Tintenbar, on the Far North Coast.
  • 1901-1905 – Opal mining begins at Lightning Ridge. The first shaft was put down around 1901 or 1902 by Jack Murray, a boundary rider who lived on a property nearby. Some time later, possibly a few months, a miner from Bathurst named Charlie Nettleton arrived and commenced shaft sinking. It was he who in 1903 sold the first parcel of gems from the field for $30, not a fiftieth of the price that could have been obtained five years later.
  • 1908 – Opal mining begins at the Grawin-Sheepyard Field in the Lightning Ridge area, increasing the importance of the opal fields in the district.
  • 1919 – Opal mining begins at Tintenbar, continuing to 1922.
  • 1920 – The Newfield opal area is discovered.
  • 1985 – Seminal work by the Geological Survey of New South Wales leads to better, more scientifically controlled exploration for opals.
  • 1989 – The Coocoran opal area is discovered in the Lightning Ridge district.
  • 1998-1999 – The estimated value of opal production in the State is about $44 million. New South Wales (and Australia) is a leading world producer of opals.

 

Sources :

  • “Opal in South Australia”, Mines & Energy Resources, SA
  • “Opal”, Qld Dept. of Mines & Energy
  • Minerals NSW
  • “Make your own Luck with Opal”, Jewellery World, June 2000. 
  • Queensland Boulder Opal Association
  • “Opals”, by Fred Ward, Gem Book Publishers, 1997.
  • “Australian Precious Opal”, Andrew Cody, 1991.

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FAQ : How can I tell if an opal is real? What is Gilson opal? What are synthetic opals? How can I tell if an opal is synthetic? What is synthetic opal? How can I identify opals? Is my opal real?

Synthetic Opal, Doublets & Triplets

Gilson opal

Synthetic (Gilson) Opal

Ever wondered if you’re getting what you paid for? Synthetic opal does exist, as well as partially man-made stones such as triplets and doublets. In this article, we give you the low-down on how to know exactly what you’re buying. (See our article on types of opal for a more detailed explanation of the difference between solids, doublets, & triplets.)

  1. Does the stone have a white body tone, or is it transparent? If so, it’s almost certainly a genuine solid, and you’re looking at a white or crystal opal. All doublets and triplets are dark in body tone because they have a black artificial backing.
  2. Look at the side of the opal – if it has distinct visible ‘layers’, it may be a doublet or triplet (i.e. not a solid opal) In this case, one of the layers will be a thin slice of opal, attached to the dark backing. A triplet will have a third layer, which is a clear, domed layer on top of the opal.
  3. Look at the back of the opal – does it look or feel like a kind of hard black or grey plastic? Triplets are often glued on to a black plastic, glass, or vitrolite backing. Doublets are a little more difficult to identify, as they often use a natural potch (black, colourless opal) or ironstone (the brown boulder opal host rock) backing. In this case, look at the side of the stone again and see if the ‘join’ between the opal and the backing is perfectly flat (i.e. the line around the circumference is perfectly straight). Most genuine solid opals have an irregularity in this area – curved or bumpy due to their natural formation – whereas a man-made stone will be perfectly flat because the two sections are flattened so they can be glued together. Be especially wary if the opal is set in jewellery and you cannot see its back or side. Even an expert will have difficult identifying a doublet set once it’s set in jewellery with the back & sides covered.
  4. Does the top of the opal look ‘glassy’? Triplets are capped with hard clear plastic or quartz, so the top of the opal reflects differently to that of natural opal. Also, if you can see through the top of the opal from a side view, you are probably looking at a triplet.
  5. Be educated before you buy. Know what real opal looks like, and compare what you have seen to what you are buying. People have been known to set coloured tinsel or foil underneath clear plastic to make an ‘imitation opal’.
  6. Synthetic solid opal can be very difficult to identify, unless you are an expert, or have a lot of experience. Look closely at the pattern – Opal created in a laboratory (Gilson opal), displays bright colours in large patches of colour. The pattern is often ‘too perfect’ and ordered, and can also often display a ‘snakeskin’ pattern. If you are still not sure, take it to a gemmologist or an opal expert.
  7. Lifting – If your opal becomes ‘cloudy’ after a while, you are probably looking at a triplet or doublet. This cloudiness happens when a triplet or doublet has been worn in water over a long period of time, causing the glue between the layers to deteriorate and allow water penetration.

Please note, triplets, doublets, or synthetic opals can be a great affordable substitute for natural opal. However, you should always be aware of what you are purchasing, to avoid being overcharged or misled.

In Summary: Always try to buy from somebody who has gemmological qualifications and offers a ‘certificate of authenticity’ with their opals. Reputable dealers are accountable to their gemmological associations and may also be members of a jeweller’s or opal association.

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Most common words are below:

  • Agitator – or ‘agi’ for short; a modified cement mixer used to wash and tumble opal dirt. Usually set up adjacent to a dam for water supply.
     
  • Amorphous – a word meaning “without form”, applied to gems and minerals that have no definite or orderly arrangement of atoms or crystal structure and have no external crystal structure.
     
  • Artesian Basin – a large body of underground water that covers some 1,750,000 square kilometres or around 676,250 square miles in the inland of Australia and occurs in many other places in the world. A huge source of water by way of bores or wells in the arid areas.
     
  • Automatic hoist – a machine which can be activated from underground to pull a bucket up the mine shaft and tip its content out; used to carry opal dirt up and dump it into a truck. 
     
  • Ballroom – a term used to describe a large cavity in an opal mine where the opal dirt has been removed in the search for opal. These ballrooms can be quite large.
     
  • Bar – a descriptive term for the way the actual opal colour forms in a nobby or piece of seam opal, usually referred to as “the colour bar”.
     
  • Black opal – the most rare and valuable type of opal. Due to its iron oxide and carbon content, black opal has a dark body tone, which gives greater intensity to the gem colour. The word ‘black’ doesn’t refer to the colours displayed by the opal – black opal comes in every colour of the rainbow. Learn more about black opals.
     
  • Blow – a formation resembling a cylindrical ‘tube’ varying in diameter from a few inches to many feet and found in the actual opal ‘level’, sometimes containing some opal fragments and made up of a whitish sandy material which is often very hard. Thought to be steam or pressure vents millions of years ago.
     
  • Blower – a machine like a giant vacuum cleaner, used to suck opal dirt from underground into a pipe, up the shaft and into a waiting truck. A more recent invention than the automatic hoist.  A blower can remove massive quantities of opal dirt.
     
  • Body tone – Opal is rated on a body tone scale from dark to light. N1-N4 is “black opal”, then N5-N6 is “semi black” and N7-N9 is “light”.
     
  • Bogger – a small vehicle with a tipping scoop on the front, used in some mines to move opal dirt from the mine face to the mine shaft for transport up to the truck.
     
  • Boulder opal – mined in Queensland, this gem forms naturally on a dark brown ironstone. Boulder opals often show colour just as well and sometimes better than black opals. Learn more about boulder opals.
     
  • Bummy – a slang term used on the opal fields to describe a stone which has been cut with large or excessive backs on them.
     
  • Buyer – someone who buys opal from miners, runners, or other buyers.
     
  • Cabochon – the domed or convex top which is shaped and polished on a finished gemstone. i.e. a non-faceted surface which is rounded and smooth all over. This technique is used in place of faceting in opal cutting.
     
  • Calibrate – to cut a stone to a regulated standard size, usually by template and the use of vernier calipers.
     
  • Carat – a unit of weight used to measure opals and other gemstones. One carat equals one fifth of a gram.
     
  • China Hat – a formation of rough opal nobby with a peaked centre, thought to be an ancient lily centre which has fossilised into opal.
     
  • Cleave or Cleavage – the ability of a gemstone or mineral to break in a certain direction usually because of its crystal structure. In opal, the cleavage plane is totally irregular and somewhat haphazard. The veins of opal in boulder opal are sometimes cleaved apart to expose the opal.
     
  • Common Opal – This term describes all opal that doesn’t have a play of colour, but rather is one or other of the base colours, e.g. white, grey, black – i.e. potch.
     
  • Coocoran – A fairly large basin that, during flood times, becomes a lake some 7 kilometres long and 3 kilometres wide. More importantly it is the name for the largest opal field of late in the Lightning Ridge area. Used to describe the large group of opal mining fields around and beyond the lake.
     
  • Crystal opal – any kind of opal that is transparent or translucent. Learn more about crystal opal.
     
  • Diaphaneity – the property of being transparent or translucent, often applied to opals when referring to crystal opals.
     
  • Digger – a hydraulic machine with a digging claw, used underground to mine opal. The miner stands at the controls of the digger while it is operating. 
     
  • Doublet – Fine slices of white or crystal opal placed on top of a dark backing, making it look like the much rarer black opal. Learn more about opal doublets.
     
  • Dopping – the technique of adhering a stone to a stick in order to handle it better during the cutting and finishing processes, using a specially designed wax.
     
  • Drill – these days many miners buy or hire a large drill to explore prospective opal-bearing ground. The drill is used to bore holes up to nine inches diameter, bringing earth and rock to the surface to be inspected for indications of opal.
     
  • Drive – the name used to describe a tunnel dug for the extrication of ‘opal dirt’, usually situated directly below the roof to a depth of about six feet.
     
  • Fire opal – this can refer to a couple of different things. Mexican Fire Opal is the only opal which is technically referred to ‘fire opal’ within the opal industry. However, the term ‘fire opal’ has also been used to describe any opal with a brilliant flash of ‘fire colours’ – i.e. red / orange colour. This is not a term which is generally used in Australia to describe opal. The term is also occasionally used less accurately to describe black opals with red colouring.
     
  • Fossils – a fossil is a remnant of what was, either whole or in part, but usually replaced by some other element, for example opal, which produces fossilised opal.
     
  • Gouge – a term used by miners to describe the action of gently picking at the face to find opal as opposed to actual digging. Gouging is done when checking for trace or when opal has been seen, so as to get it out quickly rather than waiting for the processing to be done.
     
  • Hoist – A mechanical device pioneered in Lightning Ridge that takes the dirt from the mine to the surface automatically.
     
  • Inclusion – Any material that has formed internally in opal, such as matrix, sand or even mud, and occasionally Gypsum, also known as dendrite.
     
  • Ironstone – Rock that has a rusty redish brown appearance and is composed of iron oxide, mostly a conglomerate. In boulder opal the actual opal is in or around this material, and is cut leaving the boulder host rock on the back of the stone.
     
  • Level – The name given to that strata where opal could be potentially found – commonly called opal dirt.
     
  • Mohs’ Scale – The internationally recognised scale for measuring hardness in gems and minerals.
     
  • Mullock or Mullock Heap – A term used to describe the piles of opal dirt lying on the surface on all the fields.
     
  • Nobby – a naturally lump-shaped piece of opal. The nobby form of opal is only found at Lightning Ridge. 
     
  • Opal – An amorphous non-crystalline gem mineral solidified from gelatinous or liquid silica deposited in cracks and cavities left by decaying vegetation, wood, crustaceans and bones millions of years before. Very valuable in its ‘black’ forms and containing a reasonable content of water. Chemical symbol: SiO2 plus H2 O. In higher grades of opal the water content can be as high as 10%. Refractive Index of 1.38 – 1.60 and a hardness of between 5.5 to 6.5 on Mohs’ scale. Learn more about opal.
     
  • Opal carving – a specialised method of opal cutting, used to conserve gem opal and to produce uniquely-shaped gemstones with freeform shapes and undulating surfaces.
     
  • Opal cutter – a skilled person who cuts rough or rubbed opal into cut and polished gemstones.
     
  • Opal dirt – claystones in which opal is found.
     
  • Opalised or Opalized fossil – opal which has filled a void in the earth caused by decomposed objects, in the shape of teeth, bones, shells, plants, etc. to form an opal fossil.
     
  • Orientation – a term associated with opal cutting used to descibe the skill of making the absolute best out of a rough piece of opal in terms of colour, shape pattern, etc. The art of getting the best out of a stone.
     
  • Potch – common or colourless opal – a form of non-precious opal that doesn’t contain gem colour. 
     
  • Prop – an upright log used to support the roof of an underground mine. 
     
  • Ratter – a person despised on the opal fields. A thief who steals opal from a mine, an agitator or a pile of tailings.
     
  • Rough – opal that hasn’t yet been touched by cutting equipment.
     
  • Rub – opal that has been roughly ground down or ‘rubbed’ by cutting machinery to remove gross impurities and establish a preliminary shape.
     
  • Runner – someone who sells opal to buyers on behalf of the owners of the opal, usually on a commission basis.
     
  • Saw – In terms of opal cutting, an automated diamond saw, comprising a circular blade with the outside edge coated with diamond. Used with water for lubrication and to avoid overheating of the opal.
     
  • Seam – a horizontal layer of opal in the ground. Opal is often found by miners by following a ‘seam’. Very thin seam is known as ‘trace’.
     
  • Triplet – A partially man-made stone, triplets are a paper-thin slice of opal with a dark backing, and quartz crystal capping to magnify the colour. The stone is made to imitate the much rarer and valuable black opals. Learn more about triplets.
     
  • White opal – opal with a white or light body tone, normally found in South Australia. Learn more about white opals.
     
  • Windlass – a winch used to haul opal dirt up out of the mine. These days, most miners use an automatic hoist or blower for this purpose.

 

Sources :

  • “Lightning Ridge, Walgett & District” information leaflet, p. 7.
  • “Black Opal: A comprehensive guide to cutting and orientation”, by Greg Pardey, GP Creations, 1999.

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FAQ :  How is opal valued? What makes a good opal? What are the different patterns in opal? What faults can opal have that detract from its value? What is the play-of-colour? What should I look for in a good opal? How are opals valued? Why are some opals more expensive than others?

The value of an opal depends on many factors. The type of opal, body tone, brilliance, pattern, colour bar thickness, the play of colour, and faults all play important roles in determining the value.

Other important factors include the quality of the cut & polish, and the size of the stone. When being valued, opal is carefully examined and given a price ‘per carat’. The overall carat size of the stone will then determine the price of the opal.

Opal class

First of all, it is essential to identify the type of opal which is being valued. An opal doublet or triplet can be worth considerably less than a solid opal. Doublets and triplets are an ‘assembled’ stone which only contains a very thin slice of natural opal and are therefore generally much less valuable.

Body Tone

Body tone is one of the most important factors in the classification and valuation of opals. Body tone refers to the background or the ‘underlying colour’ of the opal, which ranges from black through dark to light. Generally opals with a black or dark body tone are more valuable than those with a white, light, or crystal body tone, because a stone with a darker body tone tends to display colours more vibrantly.

Above – AOGIA 1-9 body tone scale.

Black opal is the most prized opal and may realise prices over AUD $15,000 a carat. Boulder opals also have a dark body tone. White opals have a light body tone and are generally the least valuable form of opal.

The term crystal opal refers to the ‘diaphaneity’ (transparency) of an opal, not its crystal structure, and is defined as any type of opal which is translucent to transparent. (See image, below) Some crystal opal displays colour so intense, so dark, that the opal is referred to as ‘black crystal opal.’

Semi Black Opal

Crystal Opal

Above – Black Opal, Semi-black Opal, Boulder Opal, Crystal Opal, White Opal

The play of colour

The phenomenon known as the “play-of-colour” is the brilliant range of the full spectrum of colours caused by the diffraction of white light by the internal structure of orderly arrayed spheres of silica. Red (fire) opal is generally more valuable than a mainly green opal which, in turn, is more valuable than a stone showing only blue colour. Nature does not produce a red colour as often as it does a blue or green. Red colouring is caused by larger microscopic silica spheres, whereas blue is caused by the more common small spheres.

Brilliance

Brilliance refers to the brightness and clarity of the colours displayed by opal, when the stone is viewed face-up. This ranges from brilliant , bright , to subdued or dull.

 

Boulder Opal

Above – left to right, brilliant , bright , and subdued.

Pattern

The pattern of coloured segments, forming the play-of-colour of a precious opal, is unique to every individual opal. The distinctiveness and colour displayed by these segments determines the quality of the pattern of an opal.

Excellent patterns include;

  • Harlequin, large sections of colour in which each colour segment is roughly the same size and shape, like a mosaic or chequerboard. A true harlequin pattern is extremely rare and highly sought after.
  • Flagstone, large sections of colour with straight edges, in a random pattern
  • Ribbon, narrow, parallel cascading lines of rolling colour
  • Straw, random thin strips of overlapping colour
  • Chinese Writing, thin strips of overlapping colour which resemble Chinese characters
  • Picture stones, the intriguingly unique patterns of ‘novelty’ or ‘picture’ stones, which resemble an object, landscape, animal, person, etc.

Good patterns include;

  • Floral – a random pattern of colour with good spread
  • Rolling Flash – large sections of colour which roll across the stone as it turns
  • Broad Flash – large sections of colour which flash as the stone turns
  • Pinfire – tiny points or specks of colour

Poor patterns are indistinct, and are characterised by patterns featuring Moss and Grass.

Above patterns – left to right, Harlequin, Flagstone,  Chinese Writing, Broad Flash, Straw, and Floral. ( Photos by Len Cram).

Colour bar

The thickness of the colour bar in opal is relative to the overall size and shape of the individual stone. Boulder opal typically has a very thin colour bar due to the way the opal is geologically formed. This should be taken into account when valuing the stone, however makes little difference to its appearance once set in jewellery.

Faults

Faults which can detract from the value of a finished opal are many and varied. A crack in the face can render almost worthless an opal that otherwise might have been worth a considerable amount per cart. Crazing, i.e. many small cracks in the opal’s face will also relegate the stone to worthless. Sand and various other minerals can be found as inclusions in and/or under the colour bar, and in the potch of opals. Other faults include potch lines, webbing, (grey lines) and windows (sections devoid or lacking in colour). The consistency of colours and pattern when viewed from different directions also has an influence – when a stone “won’t face”, the colour only shows through on certain angles and otherwise has little colour. The visibility of potch or brown ironstone on the surface of the stone will also lead to a drop in value.

Conclusion

All the above factors are taken into account when valuing opal, however there is no substitute for experience. Truth be known, there is no standardised or set method for valuing opals, as each opal is extremely unique in terms of pattern, brightness, and colouring (unlike diamonds, which can be more accurately valued according to a set chart of colours, clarity, faults, etc.) Always ask for a certificate of valuation / authenticity with your opal, and get a second opinion from an experienced valuer if you are concerned about the value of your stone.

Sources :

  • “Opal in South Australia”, Mines & Energy Resources, SA
  • “Opal”, Qld Dept. of Mines & Energy
  • The Australian Gemmologist, Vol21, #7, “Classification of Type 1 Natural Opal”, Joseph Schellnegger.

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