mineral

Common Opal (Green Opal)

Hydrated Silicon Dioxide (SiO2·nH2O)

Hardness: 5.5-6.5 on Mohs scale. Color: Pale green to apple green with yellowish tones. Luster: Waxy to vitreous. Crystal Structure: Amorphous (non-crystalline). Cleavage: None (conchoidal fracture). Specific Gravity: 1.9-2.3.

Hardness: 5
Color: Pale green to apple green with yellowish tones
Luster: Waxy to vitreous

Identified June 23, 2026More mineral →

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Physical properties

Formation & geological history

Formed from the precipitation of silica-rich groundwater in the fissures and cavities of volcanic rocks or sedimentary layers. It is often found in Tertiary-aged deposits. Color is typically derived from silicate inclusions or nickel content (prazopal).

Uses & applications

Primarily used for lapidary work, cabochons for jewelry, and as a popular specimen for rock collectors. It does not exhibit the 'fire' of precious opal but is valued for its color.

Geological facts

Unlike precious opal, common opal does not show 'play-of-color.' It can contain a high water content (up to 20%). The green variety is sometimes called 'Prase Opal' when colored by nickel minerals.

Field identification & locations

Field identification involves its waxy luster, conchoidal (shell-like) fracture, and inability to be scratched by a copper penny but can be scratched by a steel file. Commonly found in Madagascar, Tanzania, and the United States (Oregon/Nevada).

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