Opal is a gemological study all its own that can last a lifetime. There are many classifications of opal, far more than we can go into here. If you are going to buy and sell opals as a dealer, you will need a lot of experience in the valuation of opals. You also need to know the basics of how to identify opals as natural, from a gemological standpoint.
Perhaps most important, you will need a lot of experience in the wide varieties of types, grades and colors of opals because small variations in color or formation can account for wide variables in value, particularly if you want to appraise opals. Don’t think you are going to be able to go to The Guide, or some other industry publication, and get enough pricing information to appraise a fine quality opal. It just won’t happen. Opal appraising should be done by those who have heavy experience in the opal markets, and/or have a lot of friends that deal in opals.
Formation
Opals occur due to silicon rich ground water seeping into voids left by decaying ancient trees, or seeping into fissures of existing rock. Although other types of created underground voids can cause opalization to occur, the decaying forest and rock fissures are the places where many opals form. As the cellulose of the tree wood decays, it leaves empty cells in the wood. As the silicon-rich water seeps down into the void, it eventually dries out and leaves the silicon behind to form opals. Some of the water is retained by the silicon dioxide, which is why opals contain water, averaging between 10% and 15%. The silicon forms into small rows of round spheres that create the classic play of color, a diffraction of light that is the same effect as a diffraction grating spectroscope as seen below.
Opal can also occur in cavities in the rock where the water can congregate and dry slowly, allowing the silicon to form the opal. Examples can be seen in these Koroit Opals from Australia.
Specimens from the ISG Student Reference Collection. Courtesy of Gene McDevitt, McDevitt Opal Mining. koroit.com
Qualities
Opals occur in so many varieties and colors that it is difficult to find all of the possibilities in a book, much less a newsletter. From the Ethiopian Welo opal seen here, to the Australian opal seen above, to the Mexican Fire Opal seen below.
You should note that the term fire opal does not mean an opal with a lot of fire. It is a specific name for a variety of opal that is translucent to semi-transparent, yellow to orange in color, and is normally faceted…although can be cut en cabochon. I hear many, many jewelers incorrectly refer to nice quality crystal opals as “fire opal”. Fire opal qualities seen below.
Unusual Properties: Endless.
Every opal is unique, and the color and pattern possibilities are endless. Opals do occur showing asterism and chatoyancy. Here is a photograph of a fern type inclusion in an Ethiopian Welo Opal taken in the ISG Gem Lab.
Study specimen courtesy of opalholicsanonymous.com
Lab Created Opal
It should also be noted that opals, being a simple silicon dioxide, have been created in a lab. The finest by far is the Chatham Created Opal (below left). Easier to identify but also prevalent on the market are Gilson Created Opal (below center) and a created black opal from Russia (below right).
Gemological Properties
Source: Most important deposits in Australia, Brazil, Ethiopia, US (Nevada), and many others possible.
Chemical: SiO2 with H2O
Colors: Endless.
Wearability: Fair. Opals have a hardness that averages 6 on the Mohs Hardness Scale. It makes better necklaces and earrings than rings as far as long-term wear is concerned.
RI: 1.44 – 1.46
Birefringence: None
Optic Character: None
Specific Gravity: 2.10 average but can vary widely based on the amount of water contained in the stone.
Crystal System: Amorphous: no crystal structure
Hardness: 6 average
Transparency: Translucent to Opaque
I hope you enjoyed this fun look of how gemology plays an important part in all phases of a jewelry store operation. If you are interested in the world’s most amazing (and affordable) study of gemology and jewelry appraisal, we invite you to review our program by clicking here.
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