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Obsidian:
Obsidian is actually volcanic glass. It forms when silicate minerals melt and then cool very rapidly on the surface of the earth. It is extrusive.
The minerals cool so fast that there is no time for crystals to form. That's why obsidian is classified as non-crystalline. It has a 'glassy' texture because it really is glass.
Obsidian is usually black but, depending on the exact mineral composition, it may be clear, green or even amber.
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