mineral

Opal

Hydrated Silica (SiO2·nH2O)

Hardness: 5.5-6.5 (Mohs), Color: Multicolored (play-of-color on white), Luster: Vitreous to waxy, Crystal Structure: Amorphous (non-crystalline), Cleavage: None, Specific Gravity: 1.9-2.3

Hardness: 5

Identified April 6, 2026More mineral →

Identify your own rocks.

Get a report just like this from any photo, free.

App Store Google Play

Physical properties

Hardness: 5.5-6.5 (Mohs), Color: Multicolored (play-of-color on white), Luster: Vitreous to waxy, Crystal Structure: Amorphous (non-crystalline), Cleavage: None, Specific Gravity: 1.9-2.3

Formation & geological history

Formed from silica-rich water seeping into cracks and cavities in sedimentary or volcanic rock layers. Most commercial deposits are from the Cretaceous period (approx. 100 million years ago).

Uses & applications

Primary use in fine jewelry as gemstones. It is also a high-value collector's item and the birthstone for October.

Geological facts

Opals contain between 3% and 21% water. Famous specimens include the 'Olympic Australis' and the 'Aurora Australis.' Australia produces approximately 95% of the world's precious opal.

Field identification & locations

Identify by 'play-of-color' (iridescence) and a characteristic internal glow. Commonly found in Coober Pedy (Australia), Ethiopia, and Mexico. Collectors should look for vibrancy of fire and body tone.