Opal
Species
Rare · SiO₂·nH₂O
Amorphous silica holding a variable amount of water. Its play of colour comes from light diffracted by tiny silica spheres stacked in a regular grid. In Australia it was left by rainwater that carried dissolved silica down into cracks and layers of sedimentary rock; elsewhere it fills cavities in altered volcanic rock.
Identity
- Formula
- SiO₂·nH₂O
- Chemical class
- Silicate
- Also known as
- Precious opal (play-of-colour variety), Fire opal (orange to red variety), Black opal (dark-bodied precious opal)
Crystallography
- Crystal system
- Amorphous
- Habits
- Massive, Nodular, Botryoidal
- Fracture
- Conchoidal
Physical properties
- Mohs hardness
- 6
- Density
- 2.1 g/cm³
- Luster
- Vitreous, Waxy, Resinous
- Colors
- white, colorless, black, orange, red, yellow, blue, green
- Streak
- white
- Transparency
- Transparent, Translucent, Opaque
Geology
- Origins
- Sedimentary, Volcanic, Hydrothermal
- Host rocks
- Volcanic rock, Sandstone, shale and clay
- Associated minerals
- Chalcedony, Quartz, Calcite
- Geography
- Australia, Ethiopia, Mexico, Brazil, USA
Classification
- Rarity
- Rare
- Gem quality
- Yes
Arcane properties
- Affinity
- Arcane, Illusion
- Effects
- Illusion, Scrying
- Planar origin
- Material Plane
Sources
- M. Pasero and others, IMA Commission on New Minerals, Nomenclature and Classification, The New IMA List of Minerals, updated September 2026. International Mineralogical Association (CNMNC), 2026.
- Opal. Mindat.org (Hudson Institute of Mineralogy).
- Opal. Gemdat.org (Hudson Institute of Mineralogy).
- Opal Description. Gemological Institute of America, Gem Encyclopedia.
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