igneous
Obsidian
Obsidian (Volcanic glass, primarily SiO2)
Hardness (Mohs scale): 5.0–6.0. Color: Typically jet black, but can be brownish or reddish. Luster: Vitreous/Glassy. Crystal structure: Amorphous (non-crystalline). Cleavage: None, exhibits conchoidal fracture. Specific gravity: 2.35–2.6.
- Color
- Typically jet black, but can be brownish or reddish
- Luster
- Vitreous/Glassy
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Physical properties
Hardness (Mohs scale): 5.0–6.0. Color: Typically jet black, but can be brownish or reddish. Luster: Vitreous/Glassy. Crystal structure: Amorphous (non-crystalline). Cleavage: None, exhibits conchoidal fracture. Specific gravity: 2.35–2.6.
Formation & geological history
Obsidian is an extrusive igneous rock formed when felsic lava extruded from a volcano cools rapidly with minimal crystal growth. It is relatively young geologically as it eventually breaks down into minerals; most specimens are younger than Cretaceous age.
Uses & applications
Historically used for making extremely sharp arrowheads and blades. Currently used in jewelry, ornamental carvings, and specialized surgical scalpels due to its ability to hold an edge thinner than steel.
Geological facts
Because it is a glass and not a true mineral, it lacks a crystalline structure. It is often referred to as nature's glass. Some varieties like 'Snowflake Obsidian' contain white inclusions of cristobalite.
Field identification & locations
Identify in the field by its smooth, glassy appearance and sharp, curved (conchoidal) edges when broken. Common in volcanic regions such as the Western United States, Mexico, Iceland, and Italy. Collectors look for unique sheens or patterns.
