Opal

Physical Properties

Hardness: 5.5-6.5; Color: Milky white with iridescent play-of-color; Luster: Vitreous to waxy; Structure: Amorphous (non-crystalline); Conchoidal fracture.

Formation & Geological History

Formed from silica-rich water seeping into cracks and cavities in sedimentary or volcanic rock over millions of years, often in arid regions.

Uses & Applications

Primarily used as a gemstone in jewelry (rings, pendants) and highly valued by mineral collectors for its unique optical properties.

Geological Facts

The 'play-of-color' is caused by the diffraction of light through microscopic silica spheres. Unlike most minerals, opal is a mineraloid because it lacks a crystalline structure.

Field Identification & Locations

Identify in the field by its waxy luster, iridescent flashes (if precious), and presence in volcanic or sedimentary layers. Australia and Ethiopia are major sources.

Identified on: 5/19/2026

Mode: Standard