gemstone

Polished boulder opal

Opal, hydrated amorphous silica (SiO₂·nH₂O), likely ironstone-matrix opal

AI-generated identification · may be incorrect

Hardness about 5.5–6.5 Mohs; polished surface with strong play-of-color in blue, violet, green, and orange over brown ironstone. Opal is noncrystalline to microcrystalline, has vitreous-to-waxy luster, no cleavage, and specific gravity about 1.98–2.25.

Identified August 23, 2026More gemstone →

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Physical properties

Formation & geological history

Forms when silica-rich groundwater fills cavities and fractures, commonly in weathered volcanic rocks or ironstone; Australian deposits are mainly Cretaceous to Paleogene in age. The visible brown host is ironstone matrix.

Uses & applications

Used primarily as a cabochon gemstone and collector specimen; not generally used in construction or industry. Value depends strongly on brightness, pattern, body tone, stability, and provenance.

Geological facts

The spectral flashes result from diffraction of light by regularly arranged microscopic silica spheres, not from pigmentation. Boulder opal retains part of its natural ironstone host beneath or around the precious opal.

Field identification & locations

Identify by its irregular ironstone matrix, glassy polish, and shifting multicolor flashes when tilted; avoid prolonged heat or drying, which may cause cracking. Commonly found in Queensland, Australia, especially around Winton, Quilpie, and Yowah.