igneous
Obsidian
Volcanic glass (primary composition: SiO2)
Hardness: 5–5.5 (Mohs); Color: Jet black to dark grey; Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Cleavage: Conchoidal fracture; Specific gravity: 2.3–2.6.
- Hardness
- 5–5
- Color
- Jet black to dark grey
- Luster
- Vitreous/Glassy
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Physical properties
Hardness: 5–5.5 (Mohs); Color: Jet black to dark grey; Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Cleavage: Conchoidal fracture; Specific gravity: 2.3–2.6.
Formation & geological history
Formed through the rapid cooling of silica-rich (felsic) lava, preventing the growth of mineral crystals. Typically Cenozoic in geological age as it devitrifies over time. Formed at volcanic flow edges or where lava hits water.
Uses & applications
Historically used for projectile points and blades; currently used in scalpel blades for eye surgery, jewelry as gemstones, and in modern decorative construction.
Geological facts
Because it's a glass and not a mineral, it has no internal crystal structure. Ancient Mesoamerican cultures used it as a high-value currency and for ritual mirrors. Extremely sharp, it can be fractured down to molecular thinness.
Field identification & locations
Identify in the field by its characteristic curved (conchoidal) fractures and sharp edges. Commonly found in volcanic regions such as Oregon (USA), Mexico, and Iceland. High-quality pieces are translucent at the edges.
