igneous
Obsidian
Volcanic Glass (Amorphous Silicon Dioxide)
Hardness: 5 to 5.5 on the Mohs scale, Color: Typically jet black, but can be brown, tan, or dark green, Luster: Vitreous (glassy), Crystal Structure: Amorphous (lacks a true crystal lattice), Cleavage: Conchoidal fracture, Specific Gravity: 2.35 to 2.45
- Hardness
- 5 to 5
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Physical properties
Hardness: 5 to 5.5 on the Mohs scale, Color: Typically jet black, but can be brown, tan, or dark green, Luster: Vitreous (glassy), Crystal Structure: Amorphous (lacks a true crystal lattice), Cleavage: Conchoidal fracture, Specific Gravity: 2.35 to 2.45
Formation & geological history
Formed as an extrusive igneous rock when felsic lava extruded from a volcano cools rapidly with minimum crystal growth.
Uses & applications
Used for making arrowheads, cutting tools, surgical blades, and in ornamental jewelry and lapidary arts.
Geological facts
Because of its conchoidal fracture, obsidian can be chipped to produce microscopic sharp edges, making it sharper than high-quality surgical steel.
Field identification & locations
Found around volcanic regions with recent rhyolitic flows. Identified by its glossy, smooth surface, dark color, and sharp conchoidal edges.
