Mineral

Boulder Opal

Precious Opal in Ironstone (Amorphous Hydrated Silica, SiO2·nH2O)

Hardness: 5.5-6.5 (opal) / 6-7 (ironstone); Color: Brown host rock with flashes of green, blue, and red; Lustre: Sub-vitreous to waxy; Crystal structure: Amorphous; Fracture: Conchoidal; Specific Gravity: 2.1-2.5.

Hardness: 5
Color: Brown host rock with flashes of green, blue, and red

Identified April 15, 2026More mineral →

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Physical properties

Formation & geological history

Formed from silica-rich water seeping into cracks and voids within ironstone boulders (sedimentary rocks). Most Australian boulder opals formed during the Cretaceous period, roughly 100 million years ago.

Uses & applications

Primarily used in fine jewelry such as rings and pendants, and as high-value collectors' specimens. Due to the ironstone backing, it is more durable than other opal types.

Geological facts

Boulder opal is the second most valuable type of opal after Black Opal. It is unique because it is found and cut with the natural host rock on the back, which provides a dark base to enhance the play-of-color.

Field identification & locations

Identify by looking for thin veins of iridescent color embedded within a brown, heavy ironstone matrix. It is almost exclusively found in the Queensland opal fields of Australia (e.g., Winton, Quilpie).

Other mineral specimens

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Sandstone (with potential mineral staining/concretions)

Arenite (SiO2 based)

sedimentary

Schist

Metamorphic

Epidote

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metamorphic

Nephrite Jade

Nephrite

Mineral/Rock

Banded Gneiss (River Rock/Pebble)

Gneiss (specifically a banded variety often found as a river rock)

Metamorphic Rock

Green Apatite on Albite

Fluorapatite [Ca5(PO4)3F] on Albite [Na(AlSi3O8)]

mineral