mineral

Opal

Hydrated amorphous silica (SiO2·nH2O)

Hardness: 5.5-6.5; Color: Iridescent blue, green, and multi-colored (play-of-color); Luster: Subvitreous to pearly; Crystal structure: Amorphous; SG: 1.9-2.3

Hardness: 5
Color: Iridescent blue, green, and multi-colored (play-of-color)
Luster: Subvitreous to pearly

Identified June 21, 2026More mineral →

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Physical properties

Hardness: 5.5-6.5; Color: Iridescent blue, green, and multi-colored (play-of-color); Luster: Subvitreous to pearly; Crystal structure: Amorphous; SG: 1.9-2.3

Formation & geological history

Formed from seasonal rains that leach silica from sandstone and deposit it into cracks or voids as a silica-rich gel. As the water evaporates, it leaves behind the silica in the form of spheres. Precious opal occurs when these spheres are uniform in size and neatly stacked.

Uses & applications

Primarily used in jewelry (rings, pendants, earrings) and as a collector's gemstone. Higher quality specimens are considered semi-precious gemstones.

Geological facts

Opal is the birthstone for October. About 95% of the world's precious opal comes from Australia. The internal structure causes it to diffract light, creating the 'play-of-color' seen in the image.

Field identification & locations

Identify in the field by its distinct iridescent flash known as opalescence. Commonly found in sedimentary environments or volcanic rocks. Collectors look for vibrant colors and 'fire' within the stone.