Precious Opal

Physical Properties

Hardness: 5.5-6.5 on Mohs scale; Color: Multi-colored play-of-color; Luster: Vitreous to pearly; Structure: Amorphous (mineraloid); Cleavage: None; Specific Gravity: 1.9-2.3.

Formation & Geological History

Formed from seasonal rains that leach silica from sandstone and carry it into deep cracks. As the water evaporates, it leaves behind solid silica deposits. Much of the world's opal formed in the Cretaceous period about 65-145 million years ago.

Uses & Applications

Primarily used as a gemstone in fine jewelry (rings, pendants, earrings) and as unique collector specimens.

Geological Facts

Opal is the only gemstone that can display all colors of the rainbow through a phenomenon called opalescence, caused by the diffraction of light through tiny silica spheres. Australia produces approximately 95% of the world's precious opal.

Field Identification & Locations

Identified by its distinctive 'play-of-color' when rotated in light. In the field, it is often found within sedimentary host rocks like sandstone or ironstone. Collectors look for high 'fire' and transparency.

Identified on: 5/13/2026

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