mineral

Common Opal

Hydrated Silica (SiO2·nH2O)

Hardness: 5.5-6.5 Mohs. Color: Greenish-yellow to olive-drab. Luster: Waxy to vitreous. Crystal structure: Amorphous. Cleavage: None (conchoidal fracture). Specific gravity: 1.9-2.3.

Hardness: 5
Color: Greenish-yellow to olive-drab
Luster: Waxy to vitreous

Identified June 7, 2026More mineral →

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Physical properties

Hardness: 5.5-6.5 Mohs. Color: Greenish-yellow to olive-drab. Luster: Waxy to vitreous. Crystal structure: Amorphous. Cleavage: None (conchoidal fracture). Specific gravity: 1.9-2.3.

Formation & geological history

Formed from precipitation of silica-rich water in cavities or veins of volcanic rocks or sedimentary environments. It is often found in Tertiary volcanic deposits.

Uses & applications

Mostly used for lapidary work, cabochons for jewelry, and as decorative specimen pieces for mineral collectors.

Geological facts

Unlike 'Precious Opal', Common Opal does not exhibit a play of colors (opalescence). Oregon is famous for high-quality greenish varieties known as 'Owyhee Opal'.

Field identification & locations

Identify in the field by its distinctive waxy luster and smooth, shell-like (conchoidal) surface when broken. It feels lighter than quartz of the same size. Commonly found in basalt or rhyolite vugs.