The following opal glossary is providing the explanations
of various terms which you may encounter in the world of opal:
Agitator
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A large cement mixer, converted to a stationary machine which is used for processing opal dirt in order to find opal. Set up adjacent to a dam for water supply. |
Alluvial Opal
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Opal pieces which have been eroded from their original host rock and deposited into a younger sedimentary layer (alluvium). For example some opal regions in Queensland and some places at Coober Pedy. |
Amorphous
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Meaning without form. Materials in which the atoms and electron bonds are randomly distributed in a disordered pattern. Opposite: Crystalline. |
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Mining field in South Australia which produced stunning Crystal Opal, Matrix Opal and so called "Painted Ladies". |
Angel Stone
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Concretions of sandstone silicified by opaline silica with veins or coatings of precious opal. |
Assembled Opal
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Several small pieces of opal which have been glued together to form a single large piece. Also a form of assembled or composite opal are opal doublets and triplets. |
Ballroom
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A large chamber in an opal mine which was hollowed out in the search for opal. |
Band
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A hard siliceous band of sandstone in the clay level or at the bottom of the sandstone stratum. May carry opal. |
Bar
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See Colour Bar. |
Base Colour
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See Body Tone. |
Base Tone
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See Body Tone. |
Biscuit Band
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A flaky sandstone band in the shallow opal level which may contain opal. |
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Black opals which are transparent or transparent to translucent but still showing a body tone of N1 to N4 when viewed face-up on a white sheet of paper. |
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Black Opals show a play-of-colour within or on a black body tone compliant with N1 to N4, when viewed face-up. Most valuable variety of opal. |
Block Pattern
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General term for colour patterns in which the colour patches are fitting mosaic-like together (e.g. Flagstone Harlequin). See also Harlequin Pattern. |
Blue Opal
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Solid opal with a blue base colour. |
Bluebottle
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A blue coloured potch. |
Body Colour
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See Body Tone. |
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The background tone of an opal when it is viewed face-up while ignoring its play-of-colour. Also termed Base colour. |
Bottomed
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When a mining shaft has broken through the sandstone level and reached the opal dirt. |
Boulder Black Opal
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Boulder Opal with a distinctive dark or black body tone. This kind of opal must not be termed Black Opal but Boulder Black Opal. |
Boulder Doublet
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Opal doublet with an ironstone backing in order to resemble natural Boulder Opal. |
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Ironstone which is crossed by thin veins and specks of precious opal. |
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Opal of Type 2: A layer of opal that is still naturally attached to a backing of host rock in which it was formed (e.g. iron stone or any other rock material) since Boulder Opals occur in thin crevices and cracks in iron or sandstone boulders. This type of opal comes mainly from Queensland, Australia. |
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Colour pattern where a single colour patch covers a substantial part of the opal's face. |
Cabochon
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Gemstone cutting style which is characterized by a rounded or flat face and rounded or straight edges. Opposite: Faceted Gems which exhibit brilliance by many individually oriented facets. |
Calibrated
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Gemstones cut to standard size e.g. 10 x 8 mm standard oval cabochon. |
Carat
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Weight unit used for gems and pearls (1 carat = 0.2 grams). |
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Colour pattern resembling chaff by randomly oriented patches which are broken by fine parallel lines. |
Chinaman's Hat
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A nodule of rough opal (nobby) from Lightning Ridge which is formed in the shape of a Chinaman's hat. |
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Very rare colour pattern with criss crossed strokes of colour resembling Chinese ideographs. |
Claim
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Opal or gemstone mining area that has been claimed and may be mined exclusively by the individual claim holder. An opal claim has to be renewed annually. |
Claim Jumper
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A person who legally applies to the Mining Warden's Court to take over another miner's claim who has not fulfilled the regulations and conditions for using his claim. |
Clarity
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The degree of transparency of an opal. |
Clarity Cross
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A black cross on a white background to determine the transparency of an opal. |
Colour Bar
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Opal layer exhibiting a play-of-colour, adjacent to a layer of potch or a layer of precious opal with different colour. |
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Opal which does not show any play-of-colour. However some common opals may be valued for an attractive body colour (e.g. green, blue, yellow, orange, pink, red). In Australia common opal is termed potch. Opposite: Precious Opal. |
Concrete
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In Andamooka this is a conglomerate at the kopi-mud interface, which is the main host rock for opal. |
Conglomerate
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A coarse grained sedimentary rock composed of water worn pebbles, gravel and boulders set in a fine grained matrix of sand and clay. |
Cracked Opal
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Opal with one or more fractures which reduce its value since they may grow and the opal may break. |
Crazed Opal
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Opal with a multitude of little surface cracks which are resembling the pattern of a spider web and are greatly reducing the value. This is usually the result of an excessive drying out process when an opal is loosing large amounts of water on the surface which then may shrink at a quicker rate then the inside of the opal. Therefore opals with an extremely high water content may be more susceptible. Also very dry storing conditions over a long period of time may result in this effect. In general sedimentary opal from Australia is more stable than opals of volcanic origin so that only very few of them show this adverse effect. |
Crazing
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See Crazed Opal. |
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Opal which is transparent or transparent to translucent so that the colours may be very clear and visible below the surface, resulting in a particularly attractive three-dimensional play-of-colour. |
Crystalline
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Materials in which the atoms and electron bonds are in a regularly arranged pattern called crystal lattice. Such materials may form clearly defined crystal faces and may exhibit directional properties like cleavage or differential hardness. Opposite: Amorphous. |
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Dark Opals show a play-of-colour within or on a black body tone compliant with N5 to N6, when viewed face-up. This variety is often called Semi-black Opal. |
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The bending of waves around small obstacles and the spreading out of waves past small openings. This effect is also the cause of play-of-colour in precious opal. See also Play-of-colour. |
Directionality
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Degree of play-of-colour which an opal is losing when viewed from various angles. A very directional opal does only show good colour from a particular angle. |
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The arch of the face of an opal e.g. high, medium and low dome. See also Face. |
Dopping
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Attaching a gem to a dop stick by means of adhesive glue or dop wax in order to polish or facet the stone with greater ease. |
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A thin layer of transparent precious opal which is glued on a black backing e.g. black potch or glass and resembles Black Opal. See also Boulder Doublet. |
Duffer
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Shaft sunk on mixed level or a mine which produces no opal. |
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The polished top or front of an opal or with double-sided opals the more attractive side. The face may have a low dome, medium dome or high dome or it may be flat, undulating or baroque. |
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Related to opal this is a different word for play-of-colour. In connection with other gems Fire is referring to the colours that are produced by dispersion e.g. in faceted diamonds. |
Fire Opal
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Transparent to translucent opal with a yellow, orange or red base colour. This material usually comes from Mexico and may or may not exhibit a play-of-colour. |
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Colour pattern with variously shaped and sized patches, fitting mosaic-like together resulting in the appearance of a flagstone paving. The patches are less regular than in a chequerboard or authentic harlequin. See also Harlequin. |
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Colour pattern which consists of separated, randomly sized, shaped and distributed colour patches. |
Flaws
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Flaws like cracks or marks and inclusions that are visible from the face and which devalues the opal. |
Floater
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Piece of opal that has eroded out of its original host rock and can be found at the surface in creeks and gullies. |
Floor
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The bottom of a drive or tunnel. |
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A stone cut into an irregular shape without a specific geometrical form. Usually cut by following the natural shape of the rough stone in order to preserve weight. |
Freesize
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A stone not cut to a standard calibrated size. See also Calibrated. |
Fun Stones
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Low to medium grade Boulder Opals, Matrix Opals and specimens. |
Gem-Quality
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Opal of the highest quality range. |
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A synthetic opal originated by Pierre Gilson. See also Synthetic Opal. |
Gouge
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Mining or digging for opal using a pick. |
Gypsum
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May occur as fine needle-like crystal inclusions in opal. |
Harlequin
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General term for colour patterns in which the colour patches are fitting mosaic-like together, also called Block Patterns. There are various types of harlequin patterns e.g. Asteria Harlequin, Flagstone Harlequin, Cloverleaf Harlequin. The name Harlequin without further prefix may only be used in relation to an opal with a chequerboard harlequin pattern (authentic harlequin). This pattern is the most valuable of all and extremely rare! The colour patches are square or lozenge shaped and of similar size, resulting in a chequerboard pattern. |
Hyalite
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Clear and colourless form of common opal with a glassy appearance. If it exhibits a play-of-colour it is called Water Opal. |
Inclusions
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(1) Non-Opal particles in the stone such as sandstone spots and gypsum. (2) Remains of common opal showing in the face of the opal. |
Ironstone
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Ferrous host rock and backing material of most Boulder Opals from Queensland. Usually a combination of aluminium oxide, silicon dioxide and iron oxide. |
Jelly Opal
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Transparent opal with a particular jelly-like appearance which rather shows an indistinct, hazy opalescence than a clear colour pattern with localized patches of colour. |
Kopi
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Whitish clay-like sandstone or gypsum inclusion in opal. |
Level
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The strata in which opal could be potentially found. Also called opal level or opal dirt. |
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Light Opals are opaque or translucent to opaque and show a play-of-colour within or on a light body tone compliant with N7 to N9, when viewed face-up. The N9 category is also termed White Opal. |
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Mining town in New South Wales. Most important source of the famous Black Opal. |
Mackerel Sky
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Colour pattern resembling a sunset with clouds broken into long, thin and parallel masses. |
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(1) Porous rock from Andamooka with fine grains of precious opal often dyed dark to enhance the play-of-colour. (2) Ironstone containing fine specks and veins of precious opal. This form is correctly called Boulder Matrix Opal. |
Milk Opal
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See White Opal. |
Mohs' Scale
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A non-linear scale of hardness for minerals. Each point of the scale is represented by a specific mineral. The scale starts with the softest hardness H = 1 (talc) and ranges to the highest hardness of H = 10 (diamond). Each material with a higher hardness is able to scratch materials with a lower hardness and can only be scratched by materials with an even higher hardness. |
Mullock
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Opal dirt which has been dug out, brought up to the surface and dumped somewhere around the shaft. |
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An opal which has at least three distinct fire colours. |
Natural Joint
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Connection line in opal that is separated in two or more pieces which fit together like a jigsaw puzzle by natural formation. The connection slit may be filled with potch resulting in a single potch line or a potch web. Laymen may confuse these lines with cracks. |
New Chum
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Person who has just arrived and has no experience (green horn). |
Night Stone
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Opal which shows a very bright play-of-colour under low light. The colours often appear even brighter than under strong light. |
Nobby
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A roundish or flattish, nodular formation of rough opal almost exclusively occurring in Lightning Ridge. Nobbies are particularly known and valued for producing some of the very best opals ever found. |
Opal Dirt
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Clay which carries opal. See also Level. |
Painted Lady
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A special from of Boulder Opal from Andamooka consisting of thin films and patches of precious opal on tan quartzite. Often painted to depict a scene and sold as specimens. |
Parcel
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A more or less large pack or packs of opals, either rough, rubs or cut which are offered for sale as a lot. |
Picture Stone
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Opal which depicts a recognisable, familiar image. It might be created by precious opal only or in combination with potch and host rock. |
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Pseudomorph of opal after glauberite crystal aggregates i.e. a cluster of opalized crystals. Very rare collector's item from White Cliffs. |
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Colour pattern with many closely packed pinpoints of fire that show a columnar structure from the side. |
Pipe Opal
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(1) In Coober Pedy: Cigar shaped opal which forms when opal replaces a fossil squid backbone (belemnite). (2) In Queensland: Tubular opal which forms when fossil wood becomes opalized. |
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Colours in precious opal which are produced by the diffraction of white light when passing the gaps between the silica spheres. |
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In Australia common opal is called potch. See Common Opal. |
Potch Line
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Fine line of potch crossing a piece of opal. The individual segments of precious opal which are separated by a potch line or potch web usually show a change in structure and colour pattern resulting in a sharply defined colour patch that is bordered by the potch line followed by the adjacent colour patch. See also Natural Joint. |
Potch Web
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Fine lines of potch crossing a piece of opal in a netlike pattern. See also Potch Line. |
Ratter
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A thief who enters somebody else's claim which is known to be producing opal, usually at night or when the owner is away. |
Red on Black
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Most valuable and highly sought-after colour combination. Black opal which shows predominantly red colours. |
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Colour pattern in which the colours roll in parallel bands across the stone. |
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Colour pattern displaying colours which roll across the face of the stone upon movement. |
Roof
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The ceiling of a drive or mining tunnel. |
Rough Opal
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Uncut opal in its natural state as it is found in the mines. |
Rubbing Down
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First step in opal cutting when sandstone and waste material is removed and potch is ground off in order to expose the colour bar. |
Rubs
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Opal pieces that have been initially shaped with the colour bar partially or fully exposed but not yet finally cut and polished. |
Rush
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When people hurry in order to peg claims after the discovery of a rich opal find. |
Sandstone
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General term for host rock associated with Australian sedimentary opal. (1) In Coober Pedy: Actually a kaolinitic, silty or sandy claystone but due to weathering and silicification it is resembling sandstone in appearance. (2) In Mintabie: True sandstone composed of fine grains of clastic material (generally quartz). |
Seam Opal
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Opal which has been deposited in fissures and cracks in the host rock, resulting in opal plates or layers. Mostly found in pockets or long horizontal seams. Beside the most common form of seam opal there are further forms like nobby opal (mainly from Lightning Ridge), opal fossils, opal blobs etc. |
Sedimentary
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A geological term for material which has been transported by erosion from its original place to a secondary place where it has been settled and deposited in sediments. |
Semi-black Opal
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See Dark Opal. |
Separation Line
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See Natural Joint. |
Shin Cracker
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A hard porcelain type of opal dirt, found directly below the gravel or ironstone layer, which can fly when hit with a pick and strike the miner's shins. |
Silica
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Silicon dioxide (SiO2) that occurs in crystalline minerals (quartz) or amorphous masses (opal). |
Siliceous
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Rocks and materials containing silicon dioxide or silicate compounds. |
Skin to Skin
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Opal which is full precious opal from top to bottom i.e. no potch layers. |
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A natural single piece of opal (opal may be potch or precious opal or a mixture of potch and precious opal as long as the entire piece is composed of opal). Boulder Opals that have a backing of non-opal host rock or opal doublets and triplets which are not natural single pieces but have been glued together are NOT solid opals. |
Specking
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To search old mullock heaps for pieces of rough opal missed during mining. Particularly done after recent rain which has washed off opal dirt and may have exposed missed opal pieces. |
Straw Pattern
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Rare colour pattern in which the colours look like mixed up straws. |
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Man-made or artificial opal grown in a laboratory. These opals may be sold under names like Synthetic Opal, Gilson Opal (after Pierre Gilson the original manufacturer) and Inamori or Kyocera Opal (a Japanese manufacturer). Beside of the mentioned manufacturers there are also several unspecified Russian and Chinese manufacturers. These opals may also be termed opal imitations since they have a slightly different composition than their natural counterpart (they lack the water content usually found in natural opal and may contain some plastic or silica compounds used to cement the silica spheres in stead of the natural water). |
Tailings
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Material which is left over after dry or wet processing of opal dirt. |
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A very thin layer of transparent precious opal which is glued on a black backing e.g. black potch or glass and protected by a clear capping of quartz or glass which magnifies the play-of-colour. Resembles Black Opal. See also Doublet. |
Vertical Opal
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Opal that has formed in vertical seams rather than in horizontal seams and appears to be banded by the horizontal deposited layers of precious opal and potch. |
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Light Opals with a white body tone compliant with N9, when viewed face-up. |
Windlass
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A winch that is used to haul opal dirt up the mining shaft. |
Yowah Nut
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Small ironstone boulder found in Yowah, Queensland which contains opal either as a solid kernel or in concentric layers. |
