Chemical Formula SiO2 · nH2O
Color White, Colorless, Blue, Red, Green, Yellow, Orange, Brown, Pink, Purple, Gray, Black, Banded, Multicolored
Hardness 5.5 – 6.5
Crystal System Amorphous
Refractive Index 1.37 – 1.47
SG 1.98 – 2.25
Transparency Transparent to translucent
Double Refraction None
Luster Vitreous, pearly, waxy
Mineral Class Opal
Opals displaying play of color are known as Precious Opals, and opals lacking play of color are known as Common Opals. Gems can be cut from both the precious and common forms, but Precious Opal is the primary gem form of this stone. There are many varieties of both precious and common Opals. The most desired and beautiful form of opal is Black Opal, which is opal with a dark blue, dark green, or black background with a strong play of color. Next in importance is White Opal, which is Opal with a light colored body color (white, yellow, cream, etc.) with strong play of color. Also important is Fire Opal, or Mexican Fire Opal, which is a transparent to translucent deep-orange red form of Common Opal. Fire Opal can also display play of color, and this is a rarity called Precious Fire Opal.
Many precious Opals, besides being classified as either black or white Opals, are further classified based on the distribution and habit of their play of color. Some of these names have older sources, while some are recently coined trade names.
Opal doublets, often used in jewelry, are thin slices of precious opal glued onto a base material. Such gems are considerably cheaper than solid opals, yet provide the same play of color. Opal doublets are sometimes coated with a thin layer or dome of clear Quartz to make them more resistant to scratches (since Opal is a relatively soft gem). These are sometimes called Opal triplets.
A condition called crazing affects certain Opals, causing them to form internal cracks. Crazing is an interesting phenomenon, as it lacks consistency and is sometimes unpredictable. Although it can occur at random, its often takes place when an Opal removed from damp conditions is allowed to dry too quickly, or when an Opal is exposed to sudden intense light. Crazing may also take place when an Opal is subject to vibration, as during the cutting and polishing of a gemstone. The severity of the crazing and the time it takes to “craze” varies among gemstone. The origin is often a determining factor to its resistance to crazing, as some localities are less prone to crazing than others. A gradual drying process over months or even years can in some cases effectively stabilize the stone and allow it to be cut and polished with a substantially reduced risk of crazing.
Precious Opals are cut and polished into cabochons and used in all forms of jewelry, especially as pendants and ring centerpieces. Fire Opals are faceted into several gemstone cuts for jewelry. Boulder Opal is also a popular form which is used as jewelery, especially as cabochons. Opal, especially Common Opal, can also be carved into small ornamental figures.
Opal has an abundance of varieties. Some, such as Black Opal and Fire Opal, are universally accepted, while many others are either occasionally used or made up by dealers. The names below are names heard or seen being used by many dealers. There are many other names besides for this list, but they are rarely used and some are made up by dealers and not accepted.
Andamooka Opal – Opal from Andamooka, South Australia.
Banded Opal – Form of Opal with color bands.
Black Opal – Precious Opal with a black, dark blue, dark green, dark gray or similar darkly colored background or base color. Black Opal is the most valuable form of Opal.
Boulder Opal – Precious Opal from Queensland, Australia, found in the cracks of, or as coatings on, ironstone or sandstone boulders.
Cherry Opal – Orange-red to bright red variety of Mexican Fire Opal.
Chrysopal – Opal similar to Prase Opal, but with a golden-green color.
Claro Opal – Transparent Opal from Mexico with an intense red, green, blue, and yellow play of color.
Common Opal – Any Opal without play of color.
Contra Luz Opal – Opal where the play of color is visible only when a light source is behind the stone.
Fire Opal – Yellow-orange to red Opal.
Flash Opal – Opal with large schillers that abruptly appear and disappear as the stone is rotated.
Gilson Opal – Synthetically produced Opal created using the Gilson process.
Harlequin Opal – Opal in which the play of color is arranged in a consistent harlequin, diamond-shaped, or rectangular-shaped pattern that is very vivid. Harlequin Opal is one of the rarest and most prized forms of Opal.
Honey Opal – Transparent to translucent Opal with an orange to orange-brown, honey-colored background. It may or may not display play of color.
Hungarian Opal – Describes Opal from the old sources in Hungary (as well as other places in Europe such as the Czech Republic). This term has become corrupted and is sometimes used to describe White Opal from other locations as well.
Hyalite – Colorless, light yellow, or blue transparent variety of Opal, lacking play of color.
Jelly Opal – A transparent Precious Opal with a gelatinous appearance and a bluish sheen. Jelly Opal may also refer to a colorless, transparent Common Opal.
Lemon Opal – Opal with a lemon-yellow color.
Lightning Ridge Opal – Opal from Lightning Ridge (New South Wales), Australia. Although different forms of Opal are found at Lightning Ridge, this term often represents the high quality Black Opal found there.
Mexican Fire Opal – Form of transparent Opal from Mexico, usually with an orange or red colors. Mexican Fire Opal usually refers to the form without play of color. If it exhibits a play of color, it is known as Precious Fire Opal.
Moss Opal – Opal containing inclusions resembling moss.
Nevada Opal – Opal from the Virgin Valley (Humboldt Co.), Nevada.
Onyx Opal – Opal resembling banded Onyx.
Opal Jasper – Form of Brecciated Jasper in which the cementing material is Opal.
Opal Matrix – Thin layer of Opal on host rock (matrix).
Pinfire Opal – Opal with very small, pinhead-size color flashes.
Precious Fire Opal – Yellow-orange to red Opal (Fire Opal) with play of color.
Precious Opal – Any Opal with a play of color.
Slocum Stone – A synthetically grown Opal. Also called Slocum Opal.
Virgin Valley Opal – Opal from the Virgin Valley (Humboldt Co.), Nevada.
Wax Opal – Yellow to brown Opal with a waxy luster.
White Cliffs Opal – Opal from the White Cliffs, New South Wales, Australia.
White Opal – Precious Opal with a light colored body color, such as white, yellow, and beige. (Differentiated from Black Opal which has a dark background color.
Yowah Nut – Small, rounded form of Opal from Yowah (Queensland), Australia in a nodule embedded in ironstone. Closely related to Boulder Opal, it occurs most often as walnut-sized ironstone nodules containing pockets, veins, or sprinklings of vivid Precious Opal.Opals are occasionally treated with soaking in waxes or synthetic lubricants to enhance luster and stability. Gemstone rough Opal is usually sold in buckets of water to enhance the play of color effect which is stronger when an Opal is wet.Australia is the largest producer of Opal. Other important deposits are in Ethiopia, Sudan, Hungary, Honduras, Brazil, Mexico, and the United States (Nevada, Oregon, California, Idaho).The play of color exhibited in Opal is distinctive, and no natural gemstones can duplicate Precious Opal. Fire Opal may be similar to Topaz, Citrine, and Carnelian.
Orthoclase as a mineral is a common feldspar, but when described as a gemstone it refers to a rare transparent yellowish form of the mineral Sanidine that is found in Madagascar. Until recently, this gemstone was though to be Orthoclase, and only recently have analyses determined it as being Sanidine. Despite this, it is still called Orthoclase on the gemstone market since the name has already been established. This gemstone can have a very pleasant yellow color, sometimes with some hints of brown in it. A purer yellow color is more desirable, although champagne colored stones are also popular. A new, apple-green type has recently been discovered, though its production is very limited.
Opal TREATMENTS AND ENHANCEMENTS