mineral

Black Opal

Hydrated Silica (SiO2·nH2O)

Hardness: 5.5-6.5 on Mohs scale; Color: Dark body tone (black, dark grey) with vibrant 'play-of-color'; Luster: Vitreous to waxy; Crystal structure: Amorphous; SG: 1.9-2.3

Hardness: 5
Luster: Vitreous to waxy

Identified March 25, 2026More mineral →

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Physical properties

Hardness: 5.5-6.5 on Mohs scale; Color: Dark body tone (black, dark grey) with vibrant 'play-of-color'; Luster: Vitreous to waxy; Crystal structure: Amorphous; SG: 1.9-2.3

Formation & geological history

Formed from silica-rich water seeping into cracks in sedimentary rock (typically sandstone). Most Australian specimens formed during the Cretaceous period (approx. 100 million years ago) in Great Artesian Basin.

Uses & applications

Predominantly used as a high-value gemstone in fine jewelry (rings, pendants) and as a highly prized specimen for mineral collectors.

Geological facts

Black opal is the rarest and most valuable form of opal. The 'play-of-color' is caused by internal diffraction of light through microscopic spheres of silica arranged in a grid-like pattern.

Field identification & locations

Identify by a dark body color that enhances the brilliance of the spectral colors. Most commonly found in Lightning Ridge, New South Wales, Australia. Look for the 'potch' (common opal) backing which naturally occurs with the gem color.