mineraloid

Boulder Opal

Hydrated Silica (SiO2·nH2O)

Hardness: 5.5-6.5 (Mohs scale); Color: Brown host rock with iridescent flashes of blue, green, and gold; Luster: Vitreous to waxy; Crystal Structure: Amorphous (non-crystalline); Cleavage: None; Specific Gravity: 1.9-2.3.

Hardness: 5
Luster: Vitreous to waxy

Identified June 4, 2026More mineraloid →

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Physical properties

Formation & geological history

Formed when silica-rich water seeps into cracks and voids in ironstone or sandstone host rocks. The silica precipitates out as microscopic spheres that diffract light. Most deposits are Cretaceous in age (approx. 100 million years old).

Uses & applications

Primarily used in high-end jewelry as a gemstone, and as a highly sought-after specimen for mineral collectors.

Geological facts

Boulder opal is unique because it is mined with its natural host ironstone backing, which provides durability and a dark background that enhances the play-of-color. It is found almost exclusively in Queensland, Australia.

Field identification & locations

Identify in the field by looking for 'veins' or 'color bars' of iridescent material embedded within dark, heavy ironstone boulders. Wetting the rock helps reveal the color flashes.