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About Minerals


By Dave Jacobson for Canaveral Moonstone
June 2010

Opal is Si02•nH20, hydrated silicon dioxide. Opal is a variety of silicon dioxide which usually contains water typically in the range of 5 to 10%. Opal is a tektosilicate, low temperature hydrothermal mineral. It is commonly precipitated from silica-rich solutions. It sometimes replaces the skeletons of marine organisms. It occurs as nodules, seams and botryoidal crusts. It often impregnates porous rocks and sediments.

Some of the more common varieties are following. Common opal: colorless and not translucent. Fire opal: reddish to orange-red translucent material. Precious opal: can be any hue which displays shifting flashes of intense color which can be red, blue, orange, green, purple and more. Colorless, transparent opal is hyalite. Hyalite is usually found in botryoidal (grape like) crusts.

Hydrophane is a white, opaque, sometimes barely translucent opal which absorbs water. Wood opal replaces the mineralization in fossilized wood without destroying the structure of the material.

Opal is never found in crystal in form. It is brittle material with no cleavage plane, but has an excellent conchoidal fracture, which can be very lustrous. Hardness ranges from 5.5 to 6.5. Specific Gravity ranges from 1.99 to 2.25. The more porous material has the lower specific gravity. The gravity will increase as the opal absorbs water. When tested on a streak plate, the streak is white. It has a bright glassy to waxy luster. The color shift in precious opal is thought to be caused by the interference of reflected light due to grids of cristobalite spaced at regular intervals in the specimen.

Opal is a common mineral and is found in many locations in the world. Most of the black precious opal comes from Australia. Mexico is a source of fine fire and precious opal. In the United States precious opal is found in Nevada, Idaho and Oregon. Excellent wood opal specimens have been found in the western United States.

Opal takes its name from the Sanskrit word upala, meaning "precious stone" or "gem" and the Latin word opalus , meaning "precious stone."

The following reference materials were used in preparing this article:

Article via The Mountain Gem, July 2010