igneous
Obsidian / Volcanic Glass
Obsidian (principally SiO2, usually 70% or more)
Hardness: 5–5.5 Mohs scale; Color: typically black but can be dark green or brown; Luster: Vitreous (glassy); Crystal Structure: Amorphous (lacks crystal structure due to rapid cooling); Cleavage: Conchoidal fracture (shell-like curves); Specific Gravity: 2.35–2.6
- Hardness
- 5–5
- Color
- typically black but can be dark green or brown
- Luster
- Vitreous (glassy)
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Physical properties
Hardness: 5–5.5 Mohs scale; Color: typically black but can be dark green or brown; Luster: Vitreous (glassy); Crystal Structure: Amorphous (lacks crystal structure due to rapid cooling); Cleavage: Conchoidal fracture (shell-like curves); 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. Common in geologically young areas of volcanic activity (Cenozoic era).
Uses & applications
Historically used for edged tools and weapons (arrowheads, knives); modern use in surgical scalpel blades, jewelry (cabochons/beads), and ornamental carvings.
Geological facts
Because it is a glass and not a crystal, it is metastable and will eventually weather into perlite through hydration over millions of years. It can produce an edge many times sharper than high-quality steel surgical scalpels.
Field identification & locations
Identify by its very smooth, glassy texture and distinctive razor-sharp conchoidal fracture patterns. Commonly found in volcanic fields like the American Southwest, Iceland, Mexico, and Japan. For collectors: look for smooth curved edges when broken.
