Precious Opal

Physical Properties

Hardness: 5.5-6.5 on Mohs scale; Color: Multi-colored play-of-color on a white or translucent base; Luster: Vitreous to pearly; Crystal structure: Amorphous (non-crystalline); Cleavage: None; Specific Gravity: 1.9-2.3

Formation & Geological History

Formed from seasonal rains that leach silica from sandstone and carry it into cracks and cavities. As the water evaporates, it leaves behind a silica deposit. This process occurred mostly during the Cretaceous period (roughly 145 to 66 million years ago).

Uses & Applications

Primarily used in high-end jewelry as gemstones (cabochons), and highly prized by mineral collectors for its unique optical properties.

Geological Facts

The 'play-of-color' in opal is caused by the diffraction of light through tiny silica spheres arranged in a regular pattern. Opal is the birthstone for October and is the national gemstone of Australia.

Field Identification & Locations

Identify in the field by looking for 'potch' (common opal) with flashes of spectral colors. Found most commonly in sedimentary Rocks in Australia (Coober Pedy, Lightning Ridge) and volcanic regions in Ethiopia and Mexico.

Identified on: 4/14/2026

Mode: Standard