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Opal
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Stop Press! Stop Press! Stop Press!
The famous Virgin Rainbow Black Opal is for sale here!
Formation of opal
The best opals are found in sedimentary rock, often ironstone, with the opal lying at less than 30 metres deep.
Opals are made up of minute spheres of silica which have grown around
a central nucleus. Tiny spheres of silica weathered from overlying rock percolates through
cavities or faults where it deposits as a gel. The gradual loss of water from the gel results in hardening of the material and the formation of opal, the whole process taking hundreds of thousands of years.
Lapidary Information: Opal
| RI |
SG |
Crystal System |
Optic Character |
1.44 - 1.47
Mexican Opal as low as 1.37 |
1.98 - 2.20 |
Mainly amorphous |
SR, ADR common |
| Hardness |
Pleochroism |
Cleavage |
Light Stability |
| 5.5 - 6.5 |
None |
None |
Stable |
Toughness: Very poor
to fair depending on source and type of opal.
Dealers keep their opals for a period of time to ensure they are not susceptible
to crazing, a network of fine cracks. |
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