Opal

Opal is a hardened silica gel, usually containing 5-10 percent water. It is therefore non-crystalline, unlike most other gemstones, and may eventually dry out and crack. There are two varieties: precious opal, which shows flashes of color (iridescence), depending on the angle of viewing; and common or ‘potch’ opal, which is often opaque and displays no iridescence.

The iridescence of precious opal is due to the way the structure, a regular arrangement of tiny silica spheres, diffracts light. The larger the spheres, the greater the range of colors. Precious opal occurs in a number of color varieties. The one with a dark background is called black opal. It is common to find transparent fire opal in a volcanic rhyolite matrix.

Opal has been imitated by Slocum stone. It is a tough, man-made glass, and in 1973 Gilson made an imitation opal in the laboratory.

Occurrence

Opal fills cavities in sedimentary rocks or veins in igneous rocks. It forms stalagmites or stalactites and replaces organic material in fossil wood, shell, and bone. Australia is the main producer of opals since the 19^th century. Other localities include the USA, Czechoslovakia (Czech Republic/Slovakia), Sudan, Mexico, Brazil, and Southern Africa.

Opal Properties

• Specific Gravity: 2.10
• Refractive Index: 1.37-1.47
• Birefringence (DR): None
• Luster: Vitreous
• Crystal Structure: Amorphous
• Composition: Hydrated silica gel
• Hardness: 6

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