mineral

Precious Opal

Hydrated Silica (SiO2·nH2O)

Hardness: 5.5-6.5; Color: Play-of-color (iridescence) over white or translucent base; Luster: Subvitreous to waxy; Structure: Amorphous (non-crystalline); Cleavage: None; Specific Gravity: 1.9-2.3.

Hardness: 5
Color: Play-of-color (iridescence) over white or translucent base
Luster: Subvitreous to waxy

Identified March 25, 2026More mineral →

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Physical properties

Hardness: 5.5-6.5; Color: Play-of-color (iridescence) over white or translucent base; Luster: Subvitreous to waxy; Structure: Amorphous (non-crystalline); Cleavage: None; Specific Gravity: 1.9-2.3.

Formation & geological history

Formed from silica-rich water seeping into cracks in sedimentary rocks or volcanic cavities. As water evaporates, silica spheres deposit in an ordered grid. Most commercially significant deposits are Cretaceous to Tertiary in age.

Uses & applications

Primarily used as a gemstone in high-end jewelry (rings, pendants, earrings) and as a birthstone for October. Also valued by mineral collectors for natural specimen display.

Geological facts

Unlike most gemstones, opal is mineraloid because it lacks a crystal structure. Its internal colors are caused by the diffraction of light through microscopic silica spheres. Australia produces about 95% of the world's precious opal.

Field identification & locations

Identify in the field by the 'play-of-color' flashes when rotated. Found commonly in Coober Pedy, Australia; Ethiopia (Welo); and parts of Mexico. Collectors should look for vibrancy of color and body tone.