Mineral description : Opal

Precious opal from Australia.
Fire opal from Mexico.

Mineral class Silicates : quartz group.
Chemical formula SiO2.H2O
Crystal system Amorph.
Habitus Ball formed aggregates, crusts, stalactites, massive or earthy.
Cleavage None, conchoidal fracture.
Hardness 5.5-6.5
Density 1.9-2.5
Color Usually colorless, white to bluis. Sometimes bluish black or red. Precious varietys have a rainbow colorplay.
Streak White.
Luster Vitreous to greasy.
Occurance Created by silica rich solutions at low temperatures. Commonly around hot springs.
Notes Often fluorescent in UV light.
Insoluble in acids.
Decomposes when heated, giving off water, and may turn into quartz.
Sensitive against dehydration. It becomes fragile and could develop fractures when drying up.
Locations A rare mineral in Sweden. No locality is known of precious opal.
  • Tabergs mines. Together with calcedone.
  • Nordmark, Värmland.
  • Mossgruvan, Ljusnarsberg, Västmanland.

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Last changed : 1998-09-08