mineral
Common Opal
Hydrated Silica (SiO2nH2O)
Hardness: 5.5-6.5 on Mohs scale; Color: Greenish-yellow to tan; Luster: Waxy to vitreous; Crystal Structure: Amorphous (non-crystalline); Cleavage: None (conchoidal fracture); Specific Gravity: 1.9-2.3
- Hardness
- 5
- Color
- Greenish-yellow to tan
- Luster
- Waxy to vitreous
Identified More mineral →
Explore Common Opal in the encyclopedia →Identify your own rocks.
Get a report just like this from any photo, free.
Physical properties
Hardness: 5.5-6.5 on Mohs scale; Color: Greenish-yellow to tan; Luster: Waxy to vitreous; Crystal Structure: Amorphous (non-crystalline); Cleavage: None (conchoidal fracture); Specific Gravity: 1.9-2.3
Formation & geological history
Formed from silica-rich groundwater precipitating in the cavities and fissures of igneous or sedimentary rocks. It lacks the internal structure required for 'play-of-color' seen in precious opal.
Uses & applications
Common opal is used primarily for lapidary work, cabochons, and as collector specimens. Metaphysical uses are also common.
Geological facts
Unlike precious opal, common opal does not show flashes of color (opalescence). It is sometimes called 'potch' by miners when found alongside precious opal.
Field identification & locations
Identify in the field by its waxy luster, conchoidal (shell-like) fracture, and relatively light weight compared to quartz. It often feels 'stickier' or smoother than chert.
More like this
Other mineral specimens
Sandstone (with potential mineral staining/concretions)
Arenite (SiO2 based)
sedimentary
Schist
Schist
Metamorphic
Epidote
Epidote | Ca2(Al2,Fe3+)(SiO4)(Si2O7)O(OH)
metamorphic
Gypsum (variety Selenite or Alabaster)
Calcium sulfate dihydrate (CaSO4·2H2O)
Mineral
Nephrite Jade
Nephrite
Mineral/Rock
Granite
Granite (Phaneritic intrusive igneous rock)
igneous