mineral

Boulder Opal

Hydrated amorphous silica (SiO2·nH2O) in ironstone matrix

Hardness: 5.5-6.5 (opal), 6-7 (matrix); Color: dark brown host rock with flashes of green, blue, and pink; Luster: vitrous to sub-vitreous; Crystal structure: Amorphous

Hardness: 5
Color: dark brown host rock with flashes of green, blue, and pink
Luster: vitrous to sub-vitreous

Identified June 14, 2026More mineral →

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Physical properties

Hardness: 5.5-6.5 (opal), 6-7 (matrix); Color: dark brown host rock with flashes of green, blue, and pink; Luster: vitrous to sub-vitreous; Crystal structure: Amorphous

Formation & geological history

Formed when silica-rich water seeps into cracks in ironstone or sandstone boulders during the Cretaceous period (approx. 100 million years ago); predominantly found in Queensland, Australia.

Uses & applications

Primarily used in high-end jewelry (cabochons) and as a highly sought-after collector gemstone specimen.

Geological facts

Boulder opal is unique because it is mined with its ironstone host rock intact, making it far more durable than other types of opal while providing a natural dark background that enhances the play-of-color.

Field identification & locations

Identify by looking for thin veins of iridescent precious opal naturally bonded to a dark brown ironstone back. Found in 'boulder' formations in arid outback regions of Queensland.

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