Hydrated amorphous silica known for its internal play of color caused by diffraction of light from regularly arranged silica spheres. Color: Variable, often with play of color. Luster: Vitreous to resinous. Streak: White. Cleavage: None conchoidal fracture. Hardness: 5.5-6. Transparency: Transparent to opaque. Crystal system: Amorphous. Chemical formula: SiO₂·nH₂O. Specific gravity: 1.98-2.25. Formation: Precipitates from silica-bearing waters at low temperatures, filling cavities and replacing fossils and wood. Common geological settings: Australia 90% of precious opal , Ethiopia, Mexico, Brazil; found in sedimentary and volcanic rocks. Common look-alikes and related entries: Moonstone, Labradorite, Chalcedony.
These authoritative resources support category-level nomenclature, terminology, identification methods, and safety. When a specimen-specific safety warning appears, the public-health source listed here directly supports that precaution. They are not line-by-line citations for every other property on this entry, and natural specimens vary.
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GeoMiner
Last updated
2026-03-30T14:36:43.279027+00:00
Review status
Published; not independently peer-reviewed
IMA–CNMNC List of Mineral Names — International Mineralogical Association. Official mineral-name, nomenclature, and classification records.
Mineral ID Key — Mineralogical Society of America. Physical-property identification methods, repeat-testing guidance, and limitations.
Hydrated amorphous silica known for its internal play of color caused by diffraction of light from regularly arranged silica spheres.
How can you identify Opal?
Compare several properties rather than relying on colour alone. This entry records colour: Variable, often with play of color; lustre: Vitreous to resinous; streak: White; Mohs hardness: 5.5-6; cleavage: None conchoidal fracture; crystal system: Amorphous; transparency: Transparent to opaque. Confirm an important identification with additional field tests or qualified laboratory work.
How does Opal form?
Precipitates from silica-bearing waters at low temperatures, filling cavities and replacing fossils and wood.
Where is Opal commonly found?
Australia 90% of precious opal , Ethiopia, Mexico, Brazil; found in sedimentary and volcanic rocks.
What is Opal commonly confused with?
Opal may be confused with Moonstone, Labradorite, Chalcedony. Compare multiple diagnostic properties because colour and surface appearance can overlap.
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