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Obsidian

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Colour:
black, bluish, mahogany, ...
Glassy, fresh igneous rocks not otherwise classified

Volcanic glass - a rock rather than a mineral, it is a mixture of cryptocrystalline grains of silica minerals in a glass-like suspension, a super-cooled liquid. Obsidian is formed in the latest stage of volcanic eruptions, the silicas left over after most of the other elements and water have been used up are ejected or flow out and rapidly chilled at surface temperatures.

Note on "Transparent/Translucent Obsidian":
A lot of gem-quality water-clear variously coloured 'obsidian' has been offered for sale on the internet (in particular on auction websites) with a variety of sources listed. The material offered for sale is in fact an artificial glass mass-produced in places such as Indonesia.

Visit gemdat.org for gemological information about Obsidian.


Classification of Obsidian

Sub-divisions of Obsidian

Mineralogy of Obsidian

Essential minerals - these are minerals that are required within the classification of this rock:
GlassAn amorphous, homogeneous material with a random liquid-like structure generally formed due to rapid cooling.

Physical Properties of Obsidian

Vitreous
Diaphaneity (Transparency):
Translucent
Colour:
black, bluish, mahogany, golden, peacock, etc. - the colors due largely to refraction by microscopic bubbles (and microscopic mineral inclusions such as magnetit in "Rainbow Obsidian").
Fracture:
Conchoidal

Other Names for Obsidian

Name in Other Languages:
Simplified Chinese:火山玻璃
Spanish:Œqinolita

References for Obsidian

Reference List:
Stevenson, R.J., Dingwell, D.B., Webb,S.L., and Bagdassarov, N.S. (1995) The equivalence of enthalpy and shear stress relaxation in rhyolitic obsidians and quantification of the liquid-glass transition in volcanic processes. Journal Volcan. Geotherm. Res.: 68: 297-306.

http://members.peak.org/~obsidian/index.html (International Association for Obsidian Studies)

Internet Links for Obsidian

mindat.org URL:
http://www.mindat.org/min-8519.html
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