Opal

Opal occurs in several varieties, some of which are used as gemstones. Precious opals are white, milky blue, yellow, or black and show shifting colors like those seen in oil slicks. They are found in Hungary, Mexico, Honduras, and parts of Australia.

Nonprecious opal is widespread in igneous and sedimentary rocks and in deposits from hot springs. It also occurs in a chalky form called diatomite. A noncrystalline silicon-dioxide mineral. Formula SiO2-nH2O. Glassy or pearly luster. Colorless, white, or various colors. Transparent to translucent. Hardness 5 to 6. Specific gravity 1.9 to 2.2.