mineral

Boulder Opal

Amorphous Hydrous Silica (SiO2·nH2O)

Hardness: 5.5-6.5; Color: Brown ironstone matrix with iridescent blue/green flash; Luster: Vitreous to waxy; Structure: Amorphous (non-crystalline); Cleavage: None (conchoidal fracture)

Hardness: 5
Color: Brown ironstone matrix with iridescent blue/green flash
Luster: Vitreous to waxy

Identified June 25, 2026More mineral →

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Physical properties

Hardness: 5.5-6.5; Color: Brown ironstone matrix with iridescent blue/green flash; Luster: Vitreous to waxy; Structure: Amorphous (non-crystalline); Cleavage: None (conchoidal fracture)

Formation & geological history

Formed when silica-rich water seeps into cracks in sedimentary ironstone during the Cretaceous period (approx. 100 million years ago); found notably in the Eromanga Basin of Australia.

Uses & applications

Primarily used in high-end jewelry (cabochons, pendants) and as a collector's specimen due to its unique play-of-color.

Geological facts

Unlike solid opal, boulder opal is cut with the host ironstone left on the back, which makes the opal more durable and often provides a dark background that enhances the play-of-color.

Field identification & locations

Identified by a distinct seam of colorful opal embedded within a dense, brown, sandy ironstone matrix. While the user's location (Perth) is not a mining hub, high-quality specimens are frequently traded and found in collections across Western Australia.