gemstone

Polished boulder opal

Precious opal, hydrated amorphous silica (SiO₂·nH₂O), likely matrix/boulder opal

AI-generated identification · may be incorrect

Hardness 5.5–6.5 Mohs; glassy to resinous luster; translucent brown, yellow, red, blue, and purple play-of-color in ironstone matrix. Amorphous, non-crystalline structure with no cleavage; specific gravity about 1.98–2.25.

Identified August 30, 2026More gemstone →

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Physical properties

Formation & geological history

Forms when silica-rich groundwater fills cavities and fractures in volcanic rocks or cements ironstone, commonly during weathering. Australian opal deposits are chiefly Cretaceous to Paleogene in age.

Uses & applications

Used as a cabochon gemstone, jewelry centerpiece, and mineral-collection specimen; it is generally unsuitable for high-impact jewelry because opal can craze or fracture.

Geological facts

The shifting rainbow colors arise from diffraction through regularly arranged microscopic silica spheres, not pigment. Boulder opal retains a natural ironstone backing, which can produce striking color contrast.

Field identification & locations

Identify by visible spectral flashes that change with angle, vitreous polish, and a dark ironstone matrix; it is commonly found in Queensland, Australia, especially Winton, Quilpie, and Yowah. Avoid ultrasonic cleaners, heat, and sudden drying.