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.
Description
Occurance	Created by silica rich solutions at low temperatures. Commonly around hot springs.
Associates
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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