Opal
Real mineral · rare · SiO₂·nH₂O
Amorphous silica containing up to 20% water. Its play of colour results from diffraction of light through a regular array of silica spheres. Formed by percolation of silica-rich groundwater through sedimentary cavities.
Identity
- Formula
- SiO₂·nH₂O
- Chemical class
- silicate
- Also known as
- Precious opal, Fire opal, Black 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, hydrothermal
- Host rocks
- sandstone, basalt, ironstone
- 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
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