The name "Opal" (Opale French, Opalo Spanish or Italian) is derived from the Sanskrit word "Upala" the Greek word "Opallios", and the Latin word "Opalus", all meaning "precious stone." Opal is found in a wide range in colors that include white, light blue, green, brown, red (Fire Opal), yellow and even black. Over 90% of the world's gem-opal comes from Australia's NSW, South Australia, and Queensland territories (see Opal Mining below).
Opal began its formation 15 to 30 million years ago, during the tertiary period. Starting out as a liquid, the slurry seeped through sedimentary layers of earth, settling within impervious clays at a depth of 20 to 30 metres. Over the coarse of millions of years, the liquid slurry crystalized, becoming precious opal.
Opal has a hardness of 5.5 to 6.6 on the Mohs scale. The Toughness of Opal is Fair. Opal crystalizes in the amorphous crystal system (trigonal-trapezohedral), without a particular crystal habit. Opal grows in Irregular veins, masses, and nodules. Opal has vitreous to resinous luster. Opal has a refractive index of 1.450.
The Opal mineraloid is amorphous hydrated (water molecules) silicon dioxide, with water content of between 10% to 20%. Common opal is truly amorphous, but precious opal does have a solid structural element. Amorphous materials are naturally created by the rapidly cooling molten material. This rapid cooling process slows and suspends the material's molecules before they can pack into a tight crystalline structure.
Photo (left) Attribution - Unknown/Public Domain
Opal is usually cut into a "cabochon" which has been shaped and polished into an oval as opposed to facetted. The cabochon cut is used for most opaque gems although highly transparent fire opals can be facetted.
Precious opal is composed of microscopic hexagonal and/or cubic silica spheres. An Opal's color range is produced by the interference and diffraction of light waves passing through these silica spheres and breaking the light waves into their spectral components. The term "opalescence" is commonly misused to describe this phenomenon. The correct term for the Opal's color display or "fire" is "play of color." Over 80 to 90 percent of all opal is valueless and referred to as "potch".
Opal Mining in the Australian Outback
The primary source for opal is Australia's sedimentary sandstone, shale, and limestone deposits of the Great Artesian Basin. Many of Australia's famous opal mines such as Lightning Ridge and White Cliffs in New South Wales, Quilpie, Yowah, and Opalton in Queensland, and Andamooka and Coober Pedy in South Australia produce the finest gem-quality opal in the world.
Pineapple Opal - www.whitecliffsopal.com
Opal mining in the outback regions of White Cliffs and Coober Pedy is a chaotic and disorganized affair. With no centralized oversight, miners spread out across the plain, digging holes at random, leaving behind a strange landscape of mounds resembling the surface of the moon (above, left). The satellite view of White Cliffs tells the whole story.
Opal that is mined from Australia's Andamooka mine is considered to be some of the most durable and stable opal due to its low water content. Fire opal is found in Central America, South America and the United States (Idaho and Oregon), typically within cavities formed in volcanic lava flows.
Opal Classification
Opal's color varieties are classified as: "Black" (black body color), "Semi-Black" of "Grey" (dark body color), "Light" (between semi-black and white), "White" (very light body color), and the rare "pineapple" psuedomorphic opal from White Cliffs in NSW.
General Opal classifications are: "Black Crystal Opal" (transparent to semi-transparent, with dark body color) from Lightning Ridge, "Boulder Opal" from Quilpie in Queensland (photo above, dark ironstone matrix similar to black opal), "Crystal Opal" (transparent to semi-transparent), "Fire Opal" (translucent to transparent, with yellow, orange, or red body color), and "Matrix Opal" or "Mass Opal" (thin seams of precious opal in ironstone matrix).
An opal "doublet" consists of two separate pieces, one precious opal, and the other potch, glass or ironstone that are laminated together. An opal "triplet" consists of three separate pieces, with precious opal in the center, a clear glass top and dark potch or glass bottom that are all laminated together.
Chemical composition: SiO2·nH2O
Opal is a soft stone and therefor is occasionally treated or impregnated with oil, plastic, or wax which acts as a bonding agent. Black Opal is occasionally treated with chemicals, smoke, or dye.
Being a sedimentary stone, opal has a high water content and if allowed to dry out, it can crack or "craze". In a very dry climate, Opals should be stored in a sealed plastic bag along with a damp piece of cotton to prevent dehydration. Due to its high water content, opals are very sensitive to sudden changes in temperature (thermal shock).
Synthetic or Simulant Opal
Laboratory created opal was first introduced by the French company 'Gilson Created Opal' in 1974. In a laboratory, the opal growth process takes 14 to 18 months. Laboratory created opal has no water content, unlike natural opal, making it significantly more durable. Gilson opals exhibit a strong surface fluorescence and a "lizard skin" structure. Chatham Created Opal is another high-quality opal simulant that is physically similar to natural opal.
There is a newer jewelry-grade opal simulant that is actually a polymer-impregnated manufactured opal. This simulated opal material has an opal-like silica lattice impregnated with plastic to bond the material together. Lab-created opals are manufactured and/or distributed by Created Opal Co. in NM, ICE Industries in Downey CA, and Morion Company in Brighton MA.
Opal Inclusion Library - Gilson Opals
Chatham Created Opals
Lawsuits Ignite Furor Over Created Opal
Morion Company - Polymer-Impregnated Opals
