John Seach, Volcano Live

 Obsidian - John Seach

Obsidian is dense volcanic glass, usually rhyolite in composition and typically black in color.
Obsidian is often formed in rhyolite lava flows where the lava cools so fast that crystals do not have time to grow.
Black color of obsidian results chiefly from magnetite, Fe304.
 

Obsidian
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