Common opal
Variety SiO2·nH2O
Also called Potch
Take care.
- Sensitive to heat: Keep it away from heat and sudden changes of temperature, including hot lights and ultrasonic cleaners.
About it
Opal with no play-of-colour, often waxy and milky; miners call it potch. Far commoner than precious opal; pink, blue, and green kinds are cut as cabochons.
- Type
- Variety
- Group
- Opal variety
- Formula
- SiO2·nH2O
- Hardness (Mohs)
- 5.5–6.5
- Specific gravity
- 1.98–2.25
- Streak
- white
- Lustre
- waxy, resinous, vitreous (glassy)
- Crystal system
- Amorphous (no crystal structure)
- Colours
- white, grey, yellow, pink, blue, green, brown
- Transparency
- translucent, opaque
- Cleavage and fracture
- none; conchoidal fracture
- Under UV light
- some glows green under UV
Typical values: real specimens vary. For more, see mindat.org.