igneous
Obsidian
Volcanic Glass (Amorphous SiO2)
Hardness: 5-5.5 on the Mohs scale, Color: Typically jet black, but can be brown, tan, or green; Luster: Vitreous (glassy); Crystal Structure: Amorphous (non-crystalline); Cleavage: None (exhibits conchoidal fracture); Specific Gravity: 2.35-2.60
- Hardness
- 5-5
- Color
- Typically jet black, but can be brown, tan, or green
- Luster
- Vitreous (glassy)
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Physical properties
Hardness: 5-5.5 on the Mohs scale, Color: Typically jet black, but can be brown, tan, or green; Luster: Vitreous (glassy); Crystal Structure: Amorphous (non-crystalline); Cleavage: None (exhibits conchoidal fracture); Specific Gravity: 2.35-2.60
Formation & geological history
Formed as an extrusive igneous rock when felsic lava extruded from a volcanic vent cools rapidly with minimal crystal growth.
Uses & applications
Historically used for cutting tools, arrowheads, and weapons due to its extremely sharp edges; currently used for surgical scalpel blades, ornamental objects, and gemstone jewelry.
Geological facts
Obsidian fractures with extremely sharp edges, often producing a cutting edge only a few nanometers thick, making it sharper than high-quality steel surgical blades.
Field identification & locations
Easily identified by its dark, glassy appearance and characteristic curved conchoidal fracture patterns; found globally in areas with recent volcanic activity such as the western United States, Mexico, and Italy.
