The following are classifications used for opals:
Opal, the birthstone for the month of October, is a mineral comprised mainly of tiny precipitated spheres of silica. Orderly packing of these silica spheres can cause light entering the stone to be diffracted into a rainbow “play of colors”; and is then referred to as “Precious Opal”. Precious opal from the Klinker Opal deposit exhibits a brilliant "play of colors".
Each individual precious opal gemstone exhibits a uniquely different pattern, brightness and varied “play of color” that is unmatched by any other gemstone. Precious opal is used exclusively as gemstone material in the manufacture of jewellery.
Opal is a mineral composed of Silica with a water content SiO2.nH2O. The water content of opal can range from 3% to 20%. Opal is believed to form as a result of precipitation from low temperature silica rich water solutions. The opal forming silica precipitates in an amorphous mass of closely packed "micro-spheres" which are generally random in size and randomly oriented.
See examples of levels of brightness.
Does not display a "play of color". Incident light passing through this material emanates from the amorphous mass as random oriented white light. This is Opal the mineral (common opal) which has little value except when it occurs in a clear (transparent) form which is often used for cutting "facetted" gemstones (particularly Mexican opal). Opal Resources Canada Inc. is developing a product line to use this type of opal found in our deposit. This material is often referred to as 'jelly opal or fire opal' but when it shows a play of color it is referred to as 'crystal opal' and is then classified as Precious Opal (see picture of "Okanagan Champagne and Ale Opal").
Occasionally, opal formation conditions are such that the "micro-spheres" of silica precipitate in uniform sizes and orderly orientation. When this happens the white incident light which enters the opal is diffracted by the orderly spheres into the colors of the rainbow as it emanates from the amorphous opal mass.The play of color is therefore an optical rather than a chemical property. This play of color is the sole distinguishing feature between common opal and precious opal.
The sketch at right illustrates what a natural solid opal is. Essentially, it is a cut or rough opal with no backing or overlay. These are cut from the larger more competent pieces of precious opal. Smaller pieces and chips of opal are used in inlay jewellery. A cut solid opal is referred to as a Cabochon.
The sketch at right illustrates what a natural boulder opal is. These are cut from the more competent pieces of opal bearing rock utilizing the natural rock as part of the body of the stone. Essentially, it is an opal with the natural rock backing.
Opal Doublet (black back with Opal)
Capped Boulder Opal Doublet (Clear cap, Boulder Opal)
There are two kinds:
Solid Opal doublets are composite gemstones created by backing crystal opal (transparent to translucent precious opal) with black serpentine or other dark colored rock.
Boulder Opal doublets are composite gemstones created by adhering quartz caps onto layers of opal which are still attached to the natural host rock. Boulder opal doublets are created using similar techniques to those used to make Ammolite gemstones.
Canadian opal and diamond custom jewellery