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A Guide to Common Opal Words

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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