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Opal
Chemical Composition Amorphous silica with high water content.
Colour Colourless, white, black, gray, red, blue, orange, yellow
Refractive Index R.I. 1.40-1.47.
Durability Hardness 5.5-6.5. Brittle and heat sensitive.
Crystal Structure Amorphous
Specific Gravity 1.99-2.25.
Sources / Occurrence Major suppliers include Australia, Brazil, Mexico, Czechoslovakia (formerly in Hungary), Nevada. Occasionally found as fossilized (opalized) clamshells, snail shells, or wood.
Varieties

Mexican red or orange fire opal.

Opal is a non-crystalline form of the mineral silica.
Despite its amorphous structure, displays an amazing degree of internal organization. Most opal is more than 60 million years old and generally dates back to the Cretaceous period when dinosaurs roamed the earth.

It is found near the earth's surface in areas where ancient geothermal hot springs once flowed Million of years ago, when the deserts of central Australia were a great inland sea, silica-laden sediment deposited around its shoreline. The sea receded and disappeared to become the great Artesian basin, 30 million years ago. A lot of the silica filled cracks in the rocks, layers in clay, and even some fossils. Some of this silica became precious opal.

Opal is one of the few gemstones that is sedimentary in origin. The water in opal is a remnant of that ancient sea.