opal

Opal is a hydrated amorphous form of silica, a mineraloid prized as a gemstone for its range of colors and play-of-color. Its chemical formula is often written as SiO2·nH2O, with water content that ranges roughly 3 to 21 percent by weight.

Opal lacks a crystalline structure; it is microcrystalline or amorphous. It has a hardness of about 5.5 to 6 on the Mohs scale, a specific gravity around 1.9 to 2.2, and exhibits weak or no pleochroism. Opal is isotropic in its optical properties.

The most famous opal variety is precious opal, which displays play-of-color—a shifting rainbow of spectral colors caused by diffraction on a regular array of silica spheres about 150–350 nanometers in diameter. Fire opal can be transparent to translucent with a warm body color and may lack play-of-color.

Opal forms when silica-rich solutions fill voids and fractures in rocks, often in sedimentary basins or volcanic environments. It occurs worldwide but Australia supplies the majority of gem-quality opal, with notable deposits at Lightning Ridge, Coober Pedy, Andamooka, and Mintabie. Ethiopian Welo opal and Mexican fire opal are also well known.

Common opal refers to opaque or non‑play-of-color varieties; precious opal includes black, white, and crystal types defined by intensity of color play and body tone. Boulder opal contains opal in ironstone; potch denotes colorless to pale opal lacking play-of-color. Opals are sometimes cut as doublets or triplets, using a silica or basalt base with a transparent cap to enhance durability.

Because opal contains water, it can be sensitive to dehydration and heat; it may craze or crack if dried or overheated. Most opals used in jewelry are natural, though synthetic opals have been produced. Opal is widely regarded as Australia’s national gemstone and remains a major symbol of the country’s mineral heritage.