Gem-name guide / Gem variety

Opal

SiO₂·nH₂O

Opal is hydrated silica. Its familiar play-of-colour comes from the interaction of light with an ordered arrangement of very small silica spheres. Opal is noncrystalline and is commonly described as a mineraloid; it nevertheless has a grandfathered entry in the IMA master list.

Material or mineral family: Opal
Name classification: Gem variety
Mohs hardness: 5–6.5
Typical colour: Variable; precious opal shows play-of-colour

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Names describe materials, not the authenticity, treatment, provenance or quality of a particular specimen. Physical properties vary for mixed materials; values absent from this guide have not been estimated.

Reference snapshot: September 2026. Source dates and uncertainties are retained in the record.