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Obsidian

Obsidian is a naturally occurring volcanic glass formed when lava extruded from a volcano cools rapidly with minimal crystal growth. It is an igneous rock. Produced from felsic lava,... Wikipedia

• Category:  Volcanic glass
• Color:  usually black, sometimes green or brown, rarely yellow, orange, red or blue
• Fracture:  Conchoidal
• Mohs scale hardness:  5–6
• Luster:  Vitreous
• Specific gravity:  c. 2.4
• Optical properties:  Translucent
• Other characteristics:  Texture: Smooth, glassy
• Data Source:  DuckDuckGo