Mineraloid

Black Opal

Hydrated Silica (SiO2·nH2O)

Hardness: 5.5-6.5 Mohs. Color: Dark body tone (black, dark gray, or dark green) with play-of-color. Luster: Vitreous to waxy. Crystal structure: Amorphous. Specific Gravity: 1.9-2.3.

Hardness: 5
Luster: Vitreous to waxy

Identified March 23, 2026More mineraloid →

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Physical properties

Hardness: 5.5-6.5 Mohs. Color: Dark body tone (black, dark gray, or dark green) with play-of-color. Luster: Vitreous to waxy. Crystal structure: Amorphous. Specific Gravity: 1.9-2.3.

Formation & geological history

Formed from silica-rich water seeping into cracks and cavities in sedimentary rocks, often during the Cretaceous period (approx. 100 million years ago). As the water evaporates, it leaves behind microscopic silica spheres that diffract light.

Uses & applications

Primary use is in high-end jewelry as cabochons for rings, pendants, and earrings. It is a highly sought-after gemstone by collectors due to its rarity and vibrant flashes of color.

Geological facts

Black opal is the rarest and most valuable form of opal. The 'Aurora Australis,' found in 1938, is one of the world's most famous black opals, valued at over $1,000,000. It is the official gemstone of Australia.

Field identification & locations

Identify in the field by looking for 'play-of-color' against a dark background in sedimentary ironstone or sandstone layers. Most commonly found in Lightning Ridge, New South Wales, Australia. Collectors should look for brightness of color and a lack of 'potch' (common opal) on the face.