Absorbent Opal - Opal that is porous and soaks up water and liquids, see Hydrophane

Absorption Factor - A calculation that describes the rate or amount of absorption in an absorbent opal

Ammolite - A natural gemstone of fossilised shell that exhibits and iridescence that may be mistaken for the Play-of-colour in opal, and may be considered a “natural imitation of opal”. Found in Bearpaw Formation in Alberta, Canada.

Amorphous - A material without form, from the Greek without morphe, shape form, or possibly referring to a mineral which is non-crystalline.

Andesite - An extrusive igneous rock of intermediate composition. 

Angel Stone - An Australian opal designation having two different meanings:

1. A form of hard, white baked clay in which expansion lines have manifested themselves in a haphazard network of cracks that have become filled with precious opal. (abbrev. Ekert).

2. A material that is synonymous with the so called steel band that overlies the opal level in White Cliffs and NSW. (abbrev. Ekert).

Artificial Opal - An imitation with no known natural counterparts. 

Basalt - An extrusive igneous rock of fine crystalline basic (silica poor) composition

Black  Opal - An opal belonging to the family of opal which shows a Play-of-colour (POC) within or on a black (through to very dark grey) body tone when viewed face up.

Blue Opal - An opal showing a distinct blue body colour

Bleaching - To removing or altering a colour by means of chemical or physical agents or light.

Body Tone or Body Colour - The relative lightness or darkness of the base or background colour of an opal and used to make the distinction between Black, Dark, Light or White opal.

Bonding - The cohesion of two or more parts or layers.

Boulder Opal - Opal that is presented in one piece where the opal is naturally attached to the host rock in which it was formed and the host rock is of a different chemical composition.

Brilliance/Brightness - A description of the vividness, brightness, brilliance of the Play-of-colour in a precious opal and a grading term used to describe this characteristic.

Cachalong - A variety of common opal, which, when dry, has an opaque base (body) colour ranging from dull, pure milky white to yellowish often with a mother-of-pearl lustre and may show some porosity.

Cantera - A Spanish term used to describe Mexican type “boulder opal” where the rock included in the opal is rhyolite.

Ceraunium Opal - An ancient designation for opal meaning “thunderstones” from a Latin origin.

Cherry Opal - A dark to very dark orange to red variety of “Mexican” opal

Classification - A method of classification of various items that have similar properties and attributes.

Coating - A  layer  of  a  substance  spread  over  the  surface,  or  part  of  the  surface,  of  a  stone  for protection,  colouration,  decoration or deception a covering layer.

Common  Opal - Common opals are opal which do not show the phenomenon of Play-of-colour (POC)  

Conglomerate - A sedimentary (clastic)  composed of rounded fragments , varying from small pebbles to large boulders , in a cemented hardened clay of calcareous material, iron oxide or silica.

Colour Patches - Individual colour grains that make up the patterns in a precious opal (POC) and define a pattern

Composite Products - Artificial  products composed  of  two  or  more  previously  separate  parts  or  layers  assembled  by  bonding  or other artificial methods. Their components may be natural and/or artificial but at least one part must be opal. 

Crystal Opal - A precious opal (with POC) that is transparent through semi-transparent (not translucent) in which the Play-of-colour appears to come from the surface and within the depth of the stone. Crystal opal is colloquial Australian miner’s term, accepted by industry despite opal not having a crystalline form (mineralogy). Crystal opal may have a Black, Dark or light Body Tone (Colour).

Contra Luz Opal - A precious opal (with POC) that differs from the usual precious opal in that the POC is not visible in reflected light (usually), however the POC becomes visible in transmitted light. From the Spanish “against the light”.

Cultured - The term ‘cultured’ is only applied to ‘cultured pearls’ and no other material. The secretion of layers is caused by the metabolism of living molluscs. Cultured pearls are formations secreted in the interior of the productive molluscs.  

Cut - The style or form in which  opal and artificial products have been fashioned. 

Cutting - One of several normal lapidary practices used to transform opals.  

Dark Opal - A family of opal which shows a play-of-colour within or on a dark (grey) body tone when viewed face up.

Dendritic Opal - A common opal which is translucent to opaque and shows one or more black spots or branching fern like inclusions (dendrites). 

Diaphaneity - A gemmological and mineralogical term used to describe if a gemstone is Transparent, translucent or opaque.

Doublet - A composite  opal consisting of two parts. 

Dyeing  - Application of a dye or stain to natural materials or artificial products to alter their colour.

Embedded - To enclose, encapsulate or fix firmly to a surrounding mass. Used to describe the “embedding” of opal chips or pieces in resin material before using in jewellery.

Facetted (Cutting, Fashioning) - A process of cutting a gemstone on which facets or small flat polished surfaces have been placed, a cutting style.

Face (Opal Face) - The part of a precious opal used to determine a grading quality. E.g. “how does the opal Face”. A precious opal may also be described as “Full Faced” when the Play-of-colour (POC) completely (usually) or almost completely covers the majority of the gemstone surface being presented. 

Fire Opal  - Transparent to translucent opal having a strong predominant base (body) colour ranging from light yellow and orange through yellow orange to bright red orange and cherry red usually without POC (However POC can occur) . Often referred to as Mexican Opal, although other occurrences are known in Australia and Brazil.

Filling - A treatment used to introduce a substance that occupies a whole or part of a void.  

Fissure - A very narrow opening a fine fracture. 

Fossil Opal - Opal which has replaced shells, bones, wood or other plant and animal matter including belemnites and sauropod skeletons.

Fluid - A substance of low enough viscosity that it will flow easily. 

Fracture - An opening a crack. 

Fracture Filling - A treatment used to occupy the whole or part of a fracture with a substance, e.g. resins, oil, etc., to pervade to spread throughout to occupy completely or to make full, with the purpose of making the fracture less visible.

Gem - Another  term,  often  used  as  an  adjective,  to  describe  an  exceptional   opal  or  other gemstone noting perfection or very high quality.  

Gemstone - A natural  inorganic  or  organic  materials/substances  which  have  been  formed  completely  by  nature  without  human  interference  during  its  formation.  Gemstones  are  usually  used  in  jewellery  or  objects  d’art  due  to  a  combination  of  properties  that  provide  them  with  beauty,  rarity and relative durability. 

Grain/ Colour Grains - A single colour patch in the pattern of a precious opal that shows (POC).

Harlequin Opal - An opal pattern named from the traditional harlequin patchwork bodysuit costume of the medieval clown or jester, it is one of the rarest and most valuable patterns of POC and is a term often misused.

Homogenous - Of the same kind  or nature, consisting of similar parts, or of elements of like nature, consisting of similar parts.

Hue - A designation of colour e.g. red, orange, yellow, green, blue, violet.

Host Rock - The rock in which opal is found (i.e. hosted), also the rock which is retained as an intimate part of the gemstone when “Boulder opal” is cut and polished.

Hyalite - A common opal that is usually “water clear”, however may be slightly greenish or brownish and forms as botryoidal incrustations, sometimes known as Müller’s glass.

Hydrophane - A type of opal that, when dry is opaque (usually) white, cloudy or dull coloured but that, when immersed in water is capable of absorbing water (and other liquids) and has a varying degree of porosity. When fully absorbed the opal is transparent and may display a vibrant Play-of-colour (POC). Hydrophane derives from the Greek words for “apparent in water”.

Igneous - Rocks formed by the solidification of molten (liquid) magma

Imitations - Artificial  products that  imitate  the  appearance  of  natural ma t e r i a l s   without  having their chemical composition or their physical properties or their structure. 

Inlay - Opal jewellery where the precious opal (usually) has been cut and fitted into the setting and the surface is polished flush or smooth to the surface of the item.  

Ironstone - A sedimentary rock consisting of various sizes of clastic sediments which has a high percentage of ferruginous materials incorporated or as cement. 

Irradiated / Irradiation - Exposing   opal,  diamonds,  synthetic  stones,  pearls  and  cultured  pearls  to  any  form  of  radiation  which  is  controlled  wholly  or  partially  by  man,  usually  to  alter  the  appearance  of  the material.

Iridescence - Rainbow like colours caused by the interference of light, e.g. seen on soap bubbles. Iridescence is a difference process to the Play-of-colour (POC) seen in precious opal which is caused by diffraction.

Jelly Opal - A colloquial term for opal, which is transparent and may have a very subdued Play-of-colour, and is usually only faint bluish or bluish green. The term Jelly opal is to be discouraged from use.  

Laminated - Consisting of thin layers of opal and other substances together.

Leopard Opal - A “matrix” opal where the opal has infilled the gas vesicles within an igneous rock such as “vesicular basalt”.

Light Opal - Is the family of opal which shows a Play-of-colour (POC) within or on a light (very light grey to white) body tone when viewed face up. 

Light Crystal Opal - Is part of the family of light opal which is transparent to translucent and shows a Play-of-colour (POC) within or on a light (very light grey to white) body tone when viewed face up. 

Lightning Ridge Opal - Is opal which originates from the Lightning Ridge Mining district of NSW in Australia and is one of the predominant opal mining areas. The Lightning Ridge district is renowned world-wide as it produces the most valuable Black opal.

Matrix Opal - Is opal presented in one piece where the opal is intimately diffused as infillings of pores or holes or between grains of the host rock in which it was formed.

Metamorphic - A geological term describing rocks which have been changed or altered by heat or pressure or both.

Micro-crystalline - Having its constituent crystalline grains visible by an optical microscope.

Milk/Milky Opal - A “traditional” term for opal which is to be discourage from use and refers to opal with an opaque white/light body colour (tone) which may or may not show a Play-of-colour (POC)

Mineral - A naturally occurring chemical compound, usually of crystalline form.

Mosaic/Mosaic opal - A tessellated composite opal in which small pieces “tiles” of opal (either natural, synthetic or imitation) have been arranged together to imitate a natural opal or produce a picture.

Natural Materials - Materials  that  are  completely  formed  by  nature,  without  human  intervention  during  its  formation,  that  may  subsequently  be  modified  by  normal  lapidary  practices and  or  altered by a treatment that require general or specific information. 

Non-Precious Opal - The opposite of precious opal usually described as either “Potch opal” or “Common opal”.

Normal Lapidary Practices - Methods  used  to  fashion  gemstones  and  artificial  products  which  include  cutting, sawing, grinding, faceting, polishing, carving, engraving, drilling and cleaning. 

Oiling - Filling opal fissures and/or fractures with agents such as cedar wood oil, paraffin oil, etc. to make the fissures and fractures less visible.

Opalescence - An iridescent light effect due to the scattering of light as seen in “opal glass” and a term not to be confused or used with the Play-of-colour seen in precious opal.

Opaque - A term used to describe a material which does not transmit light.

Pineapple Opal - A pseudomorph of opal that resembles a pineapple in appearance and is considered to be a replacement of minerals such as Ikaite, Gypsum or Glauberite and found in White Cliffs NSW, Australia.

Pipe Opal - Opal found as long cylindrical or tubular forms and thought to be opal replacement of tree roots or similar material.

Play-of-colour (POC) - The reflected (usually) light phenomenon that is exhibited by Precious opal.

Permeate - The filling of fissures and/or fractures with oil, wax, resin, polymer or other fluid substances, other than glass to diminish their appearance.

Potch Opal - An Australian colloquial name for opal that has no Play-of-colour, also the name given to opal in the present nomenclature differing from common opal in order to distinguish the origin of a sedimentary opal environment

Precious  Opal - Precious opal is the name generally given to opal which shows the phenomenon of Play-of-colour (POC). 

Pseudomorph - A mineral having the characteristic outward form of another species, object or shell.

Quartzite - A metamorphic rock originally formed from quartz sandstone and usually associated with precious opal from Andamooka in South Australia.

Reconstructed  Opal - Artificial  products manufactured  by  fusing  ground  pieces   of   natural  stones  to form a coherent whole.

Reconstructed Imitation Opal - Artificial  products  manufactured  by  fusing  ground  pieces   of   imitation  stones  to form a coherent whole.

Rhyolite - An fine grained extrusive igneous rock high in silica content and often associated with opal occurrences in Mexico.

Sandstone - A clastic sedimentary rock containing sand sized particles often mainly of quartz.

Seam Opal - Opal that forms in thin (usually) layers in the rock strata of the opal fields.

Sedimentary - An environment where sedimentary rocks are formed form the deposition and solidification of sediments.

Semi-Black Opal - An historical term for medium grey body colour opal that should not be used.

Silica - A compound made of Silicon (Si) and Oxygen (O) with the formula SiO2.

Stability - A measure of the ability of  opal to maintain their appearance under normal wear and care. 

Stabilisation - The process of making a material stable and often related to the application of plastics and resins to bind small pieces of material together, see also re-constructed.

Stones - Natural  materials  and  artificial  products  used  in  jewellery  or  objects- d’art,  with  the exception of metals. 

Synthetic - A gemstone or material that is man-made.

Trachyte - A volcanic igneous rock usually with a porphyritic texture.

Trade Codes - List  used  within  the  trade,  consisting  of  one  or  more  words,  for  labelling  the  quality. 

Translucent - Transmission of light only in a diffuse way between, transparent and opaque.

Treated Opal - Opals  that  have  been  altered  to  change  their  appearance  and  or  durability. 

Triplet Opal - A composite gemstone consisting of three pieces, for opal a laminate of opal glued to a black background material and covered over the top with a transparent cap.

Untreated - A material that has not been treated.

Vug - A small unfilled cavity in a rock often lined with minerals and crystals.

Waxing - A treatment involving the application of a colourless wax or similar products to, or near, the surface of opal, usually used to hide crazing cracks or similar defects.

White Opal - Opal with a light or white body tone/colour.