Opale

Opale, known in English as opal, is a hydrated amorphous form of silica (SiO2·nH2O) that lacks a crystalline structure. It is valued for its ability to display play-of-color, a diffraction effect that creates shifting flashes of rainbow colors when viewed from different angles. The water content of opal typically ranges from about 3 to 21 percent by weight, which makes the mineral relatively delicate and capable of dehydration or cracking if exposed to extreme conditions.

Opal forms when silica-rich solutions fill voids in rocks and later precipitate into a gel that hardens into a solid. The resulting microstructure consists of silica spheres arranged in a regular pattern; the size and arrangement of these spheres determine the range and intensity of colors observed. Australia is the dominant source of opal, with notable producing regions including Lightning Ridge (black opal) and Coober Pedy, among others. Other significant sources include Ethiopia, Mexico, Brazil, and the United States.

Two broad categories are commonly recognized: precious opal, which exhibits vivid play-of-color, and common opal, which lacks this phenomenon and is typically milky or uniformly colored. Fire opal refers to opal with a warm, translucent to transparent body color and may or may not show play-of-color. Within precious opal, famous varieties include black opal, crystal opal, and boulder opal, each defined by body color and the presence of host rock in the specimen.

Care and handling are important: opal is relatively soft (about 5.5–6.5 on the Mohs scale) and can crack if dried out or exposed to rapid temperature changes. Cleaning with mild soap and water is generally safe, and careful storage helps retain its appearance. Market value depends on play-of-color, pattern, body color, transparency, and origin.