gemstone

Polished boulder opal

Opal (hydrated amorphous silica, SiO₂·nH₂O), likely precious opal

AI-generated identification · may be incorrect

Smooth, tumbled oval with yellow-orange body color and greenish play-of-color; opal is amorphous (no crystal structure), Mohs hardness 5.5–6.5, vitreous to waxy luster, and no cleavage. The image alone cannot confirm whether the color is natural or treated.

Identified October 13, 2026More gemstone →

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Physical properties

Formation & geological history

Silica-rich water deposited gel in rock fractures or cavities, later hardening into opal; deposits occur in sedimentary and volcanic settings, generally of varied geological ages. Boulder opal forms as opal veins or patches within ironstone.

Uses & applications

Cut and polished for jewelry and collecting; lower-grade material may be used for decorative objects.

Geological facts

Boulder opal is commonly left attached to its ironstone host, which provides a natural backing and can create striking color patterns.

Field identification & locations

Look for shifting flashes of spectral color as the stone moves; a single photograph cannot establish provenance or distinguish natural color from enhancement. Boulder opal is especially associated with Queensland, Australia.