mineral

Black Opal

Amorphous hydrated silica (SiO2 nH2O)

Hardness: 5.5-6.5; Color: Dark body tone with vibrant play-of-color; Luster: Subvitreous to pearly; Structure: Amorphous (tightly packed silica spheres); Specific Gravity: 1.9-2.3

Hardness: 5
Color: Dark body tone with vibrant play-of-color
Luster: Subvitreous to pearly

Identified June 14, 2026More mineral →

Identify your own rocks.

Get a report just like this from any photo, free.

App Store Google Play

Physical properties

Hardness: 5.5-6.5; Color: Dark body tone with vibrant play-of-color; Luster: Subvitreous to pearly; Structure: Amorphous (tightly packed silica spheres); Specific Gravity: 1.9-2.3

Formation & geological history

Formed from seasonal rains that leach silica from sandstone into deep fissures. As the water evaporates, it leaves behind a silica deposit. Most Australian black opals date to the Cretaceous period (approx. 100 million years ago).

Uses & applications

Primarily used in high-end jewelry as gemstones (rings, pendants) and as investment-grade collecting specimens.

Geological facts

Black opal is the rarest and most valuable type of opal. Lightning Ridge in New South Wales, Australia, is the world's premier source. Its play-of-color is caused by the diffraction of light through microscopic silica spheres.

Field identification & locations

Identify by looking for a dark grey to jet black 'body tone' that enhances the vividness of the rainbow colors. Check for 'potch' (common opal) backing which is natural in many specimens. Often found in sedimentary nodules locally called 'knobbies'.