Silica polymorphs group - amorphous

Formule chimique : SiO2-nH2O
Système cristallin : Amorphous
Etymologie : From Sanskrit upala, meaning precious stone
Dureté : 5 – 6
Densité : 2
Propriétés physiques : Vitreous to resinous sheen. Colourless, white, iridescent, black, orange-pink (fire opal). Fluorescence. Conchoidal fracture. No cleavage. Transparent to translucent.
Propriétés chimiques : Amorphous silica hydrated to spherulites.
Formule chimique : SiO2-nH2O
Habitus : In tonsils and strands, in pseudomorphosis of some fossils.
Gîtologie : Mineral of siliceous volcanic rocks or in veins in sandstones.
Determination : The gem varieties are easy to determine.
Utilisation : in jewellery

The main opal deposits are located in Australia (famous for its high quality iridescent opals), Mexico (famous for its fire opals), Guatemala, Honduras and Brazil.
In France, opal can be found in the Puy de Dôme, in the Haute-Loire (Roche Lambert), in the Ardèche and in the Paris basin.

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