mineral

Black Opal

Hydrated Silica (SiO2·nH2O)

Hardness: 5.5-6.5 (Mohs); Color: Dark body tone (grey to black) with 'play-of-color'; Luster: Vitreous to waxy; Structure: Amorphous (non-crystalline); Specific Gravity: 1.9-2.3.

Hardness: 5
Color: Dark body tone (grey to black) with 'play-of-color'
Luster: Vitreous to waxy

Identified March 25, 2026More mineral →

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Physical properties

Hardness: 5.5-6.5 (Mohs); Color: Dark body tone (grey to black) with 'play-of-color'; Luster: Vitreous to waxy; Structure: Amorphous (non-crystalline); Specific Gravity: 1.9-2.3.

Formation & geological history

Formed from silica-rich water seeping into cracks and cavities in sedimentary rocks, such as sandstone or ironstone. As the water evaporates, it leaves behind microscopic silica spheres. Most opal-bearing rocks range from the Cretaceous to the Tertiary period.

Uses & applications

Primarily used as a high-end gemstone in jewelry (rings, pendants, earrings) and as a valuable specimen for collectors of rare gems.

Geological facts

Black opal is the rarest and most valuable variety of opal. The 'play-of-color' is caused by the diffraction of light through the internally ordered arrangement of silica spheres. Lightning Ridge in Australia is the most famous source for the finest black opals.

Field identification & locations

Identify by its dark body color contrasted against vibrant internal flashes of red, blue, or green. In the field, look for 'potch' (common opal) in sedimentary layers. Collectors should look for 'pattern' and 'brightness' grading.