mineral
Boulder Opal
Amorphous Hydrous Silica (SiO2·nH2O)
AI-generated identification · may be incorrectHardness: 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
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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.
