mineral

Common Opal (Potch)

Hydrated Silica (SiO2·nH2O)

Hardness: 5.5-6.5 on Mohs scale; Color: Pale green, white, tan, and brown; Luster: Waxy to vitreous; Crystal Structure: Amorphous (non-crystalline); Cleavage: None, exhibits conchoidal fracture; Specific Gravity: 1.9-2.3

Hardness: 5
Color: Pale green, white, tan, and brown
Luster: Waxy to vitreous

Identified July 13, 2026More mineral →

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Physical properties

Formation & geological history

Forms from the precipitation of silica-rich water in the cavities and cracks of volcanic rocks or sedimentary layers. Unlike precious opal, common opal lacks the internal structure to diffract light into a play of color.

Uses & applications

Primarily used for lapidary rough, specimen collecting, and occasionally carved into beads or decorative objects. Green common opal is often marketed as 'pistachio opal' or 'mint opal'.

Geological facts

Common opal is much more abundant than precious opal. It is technically a mineraloid rather than a mineral because it does not have a crystalline structure. The green color in specimens like this is often caused by inclusions of nickel or iron silicate minerals.

Field identification & locations

Identify in the field by its waxy luster, conchoidal (shell-like) fracture, and relatively light weight for its size. It feels 'softer' or more plastic-like than quartz. Commonly found in volcanic regions of Oregon, Idaho, and Nevada in the US, as well as Tanzania and Australia.