opaalin

Opal is a hydrated amorphous form of silica (SiO2·nH2O) whose water content typically ranges from about 3% to 21%. Unlike crystalline quartz, opal lacks a long-range ordered structure. Its most notable feature is play-of-color, a spectral iridescence produced when light diffracts through the tiny, closely packed silica spheres within the mineral. The size and arrangement of these spheres determine the colors seen.

Common terms used for opal varieties include precious opal, which displays play-of-color, and common opal, which does not. Fire opal refers to opal with a vivid body color, often orange or red, that may or may not show play-of-color. Opals vary from transparent to opaque and come in a range of body colors, sometimes with a milky or milky-blue appearance.

Most commercial opal originates from Australia, notably regions such as Coober Pedy and Lightning Ridge. Other important sources include Ethiopia (notably Welo opal), Mexico (fire opal), and smaller deposits in several countries. Opal forms in silica-rich environments where groundwater fills voids in rock, vesicles in volcanic rocks, or fossilized structures, and can replace organic material in a process called opalization.

Physical properties of opal include a hardness of about 5.5–6.5 on the Mohs scale and a specific gravity around 1.98–2.25. It has a relatively high index of refraction for a non-crystalline mineral. Opal is sensitive to dehydration and temperature changes, which can cause cracking or crazing; it is also relatively brittle and should be handled with care.

In jewelry, opal is often cut as natural stones or prepared as doublets or triplets, sometimes impregnated with resins to improve stability.