Opal
Mineral SiO2·nH2O
Also called Hydrophane, Opal-CT, and Opal-A
Take care.
- Sensitive to heat: Keep it away from heat and sudden changes of temperature, including hot lights and ultrasonic cleaners.
About it
Hydrated silica with no crystal shape, softer and lighter than quartz; precious opal flashes spectral colours. Australia, Ethiopia, and Mexico are the main sources. It can crack if it dries out or heats quickly.
- Type
- Mineral
- Group
- Silicate (tectosilicate), silica group
- Formula
- SiO2·nH2O
- Hardness (Mohs)
- 5.5–6.5
- Specific gravity
- 1.9–2.3
- Streak
- white
- Lustre
- vitreous (glassy), resinous, waxy, pearly
- Crystal system
- Amorphous (no crystal structure)
- Colours
- white, colourless, yellow, orange, red, brown, blue, green, black, play-of-colour
- Transparency
- transparent, translucent, opaque
- Cleavage and fracture
- none; conchoidal fracture
- Under UV light
- often green under shortwave UV (hyalite); common opal may glow white or green
Typical values: real specimens vary. For more, see mindat.org.