mineraloid

Opal

Hydrated silica (SiO2 "NH2O)

Hardness: 5.5–6.5•Color: Milky white with play-of-color•Luster: Subvitreous to waxy•Structure: Amorphous (non-crystalline)•Cleavage: None•Specific Gravity: 1.9–2.3

Hardness: 5

Identified June 23, 2026More mineraloid →

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Physical properties

Hardness: 5.5–6.5•Color: Milky white with play-of-color•Luster: Subvitreous to waxy•Structure: Amorphous (non-crystalline)•Cleavage: None•Specific Gravity: 1.9–2.3

Formation & geological history

Formed from silica-rich water seeping into cracks and cavities in sedimentary or volcanic rock. Over millions of years, the water evaporates, leaving behind solid silica. Most Australian deposits date back to the Cretaceous period.

Uses & applications

Primarily used in jewelry as gemstones (cabochons), and highly valued by mineral collectors for its unique optical properties.

Geological facts

Opal is unique because it is an amorphous mineraloid rather than a crystal. Its play-of-color is caused by the diffraction of light through tiny, organized silica spheres.

Field identification & locations

Identify by its iridescent play-of-color. Commonly found in Coober Pedy (Australia), Ethiopia, and Mexico. Collectors should look for vibrancy, pattern, and the absence of 'crazing' or cracks.