Opal
Mineraloid (hydrated silica) · SiO₂·nH₂O
Opal is a hydrated form of silica famous for its “play-of-colour”, flashing rainbow hues, though common opal is solid-coloured.
What is opal?
Opal is hydrated silica containing several percent water, and technically a mineraloid because it lacks a crystal structure. Precious opal diffracts light through microscopic silica spheres to produce shifting rainbow flashes (play-of-colour); common opal (“potch”) is a solid colour. It fills cavities and replaces wood and shells to form fossils.
Properties
- Chemical formula
- SiO₂·nH₂O
- Category
- Mineraloid (hydrated silica)
- Hardness (Mohs)
- 5.5–6.5
- Crystal system
- Amorphous
- Lustre
- Vitreous to resinous
- Streak
- White
- Colour
- White, black, fire-orange, with play-of-colour
- Cleavage / fracture
- None; conchoidal fracture
How to identify opal
- →Play-of-colour: flashing rainbow flecks that shift as you tilt it (precious opal).
- →Glassy to resinous lustre; conchoidal fracture.
- →Softer than quartz, hardness 5.5–6.5.
- →Can contain water and may craze (crack) if it dries out.
Where opal is found
Australia (Coober Pedy, Lightning Ridge) is the classic source; also Ethiopia, Mexico (fire opal) and Brazil.
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