gemstone

Polished boulder opal

Opal, hydrated amorphous silica (SiO₂·nH₂O), likely Australian boulder opal in ironstone matrix

AI-generated identification · may be incorrect

Hardness about 5.5–6.5 Mohs; polished oval cabochon showing mottled blue, green, yellow, and orange play-of-color against a brown host. Opal is amorphous and has a vitreous-to-waxy luster; the image does not reveal precise composition or quality.

Identified October 4, 2026More gemstone →

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Physical properties

Formation & geological history

Silica-rich water deposited opal in cracks and cavities in ironstone; the pictured material is consistent with Australian boulder opal, formed during Cretaceous weathering and groundwater activity.

Uses & applications

Used in cabochon jewelry and as a collectible gemstone; ironstone-backed boulder opal is commonly fashioned without removing its natural host rock.

Geological facts

Opal’s shifting colors result from light diffraction by regularly arranged microscopic silica spheres, not pigment.

Field identification & locations

Look for changing spectral flashes as the stone is tilted, often alongside brown ironstone matrix; major sources include Queensland, Australia. A photo alone cannot confirm the exact opal variety.