Australian Black Opals and Opal, in general, is a beautiful, but paradoxical
gemstone . . . and one of the most fascinating.
Opal is a form of quartz . . . then, it is not a form of quartz. Quartz is very common, yet
has many rare and precious gem varieties. Opal itself has numerous varieties. It is the most
colorful gemstone, but some forms are colorless. It can be very bright and beautiful, and it
can be dull and dead. It is best known for its blazing flashes of color, but some varieties
have no flashes of color at all . . . and are still opals. It can be black, and it can be
white. Its best known attribute, the brilliant flashes of multiple colors, are not called
opalescence, but irridescence. Some people think opal is unlucky, but it is one of the most
valuable and desirable of all gems.
Forms of Quartz
Opal is a variety of quartz. Quartz in turn is silicone dioxide, one of the commonest minerals
on earth. Quartz exists in a number of different forms, ordinary sand is one form, but there
are numerous gemstone forms of quartz.
Opal as a Gel
Actually because opal is a gel, it is, strictly speaking, not a form of quartz. Quartz is a
crystalline form of silicon dioxide, opal is a solid gel. However because the chemical formula
is the same except that opal is hydrous, that is it contains some water which is chemically
attached to the silicon dioxide molecules.
Ancient Sources
Opals were known and mined in Roman times. The Roman mines were in and around Cervenica,
previously in Czechoslovakia, although we have now lost track of all the recent changes in
Balkan geography.
Durability
Opal is one of the softest gemstones and should be enjoyed, but owned with reasonable care.
Technical Characteristics
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Chemical Composition:
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SiO2nH2O - Hydrous Silicon Dioxide
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Hardness:
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5.5 to 6.5
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Specific Gravity:
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1.95 - 2.20
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Refractive Index (R.I.):
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1.44 - 1.47
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Optical Character:
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Isotropic
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Crystal Structure:
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Gel
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