opalos

Opals are hydrated amorphous silica minerals (SiO2·nH2O) characterized by a unique play-of-color—the iridescent flashes that appear as the stone is moved. This optical effect arises from the arrangement of microscopic silica spheres that diffract light.

Most opal forms when silica-rich solutions fill porosity and voids in rocks, later dewatering and solidifying. The largest and most diverse producers are in Australia, with notable deposits at Coober Pedy, Lightning Ridge, and Mintabie; other sources include Ethiopia, Mexico, and the United States.

Common categories include precious opal, which exhibits play-of-color, and common opal (also called potch), which lacks the color play. Fire opal is a translucent to transparent opal that may be colorless or colored with red, orange, or yellow body color; some fire opals also show play-of-color. Black opal, boulder opal, and matrix opal refer to opals with dark body color or host-rock associations.

Opals range in appearance and value according to play-of-color, brightness, pattern, body color, and clarity. They are relatively soft (Mohs 5.5–6.5) and contain water, making them sensitive to drying, heat, and rapid temperature changes. Cleaning with mild soap, avoiding ultrasonic cleaning, and storing in moisture help preserve them. Some opals are treated, stabilized, or mounted as doublets or triplets; synthetic opals are also produced.

In jewelry, opals have long been prized; Australia regards opal as its national gemstone, and the October birthstone designation appears in some traditions.