igneous
Obsidian
Obsidian (Volcanic Glass)
Hardness: 5–5.5 (Mohs scale); Color: Black, dark brown, or grey; Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Cleavage: None (conchoidal fracture); Specific Gravity: 2.35–2.6
- Hardness
- 5–5
- Color
- Black, dark brown, or grey
- Luster
- Vitreous/Glassy
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Physical properties
Hardness: 5–5.5 (Mohs scale); Color: Black, dark brown, or grey; Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Cleavage: None (conchoidal fracture); Specific Gravity: 2.35–2.6
Formation & geological history
Formed as an extrusive igneous rock when felsic lava extruded from a volcano cools rapidly with minimal crystal growth. It is relatively young in geological terms as it eventually becomes unstable and devitrifies (crystallizes) over millions of years.
Uses & applications
Historically used for arrowheads and cutting tools; currently used in jewelry (semiprecious gemstone), scalpel blades for high-precision surgery, and as an ornamental stone in home decor.
Geological facts
Obsidian is technically a glass rather than a true mineral because it lacks a crystalline structure. Native American tribes traded it extensively across North America for use as weaponry and ritual objects.
Field identification & locations
Identify by its characteristic conchoidal (curved/shell-like) fracture patterns and sharp edges. It is commonly found in volcanic regions such as the Cascades in the US, Iceland, Mexico, and Japan. Collectors should look for translucent edges when held to light.
