igneous
Black Obsidian
Obsidian (Volcanic Glass, rich in SiO2)
Mohs hardness: 5 to 5.5; Color: Jet black, opaque to translucent on thin edges; Luster: Vitreous (glassy); Fracture: Conchoidal with sharp edges; Structure: Amorphous (non-crystalline glass)
- Hardness
- 5 to 5
- Color
- Jet black, opaque to translucent on thin edges
- Luster
- Vitreous (glassy)
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Physical properties
Mohs hardness: 5 to 5.5; Color: Jet black, opaque to translucent on thin edges; Luster: Vitreous (glassy); Fracture: Conchoidal with sharp edges; Structure: Amorphous (non-crystalline glass)
Formation & geological history
Formed when felsic lava extruded from a volcano cools rapidly with minimal crystal growth. Rich in silica, it is found in geologically young volcanic environments.
Uses & applications
Historically used for knapping arrowheads, blades, and scrapers; modern uses include decorative carvings, polished gemstones, surgical scalpel blades, and lapidary art.
Geological facts
Obsidian produces an edge sharper than high-grade surgical steel because its structure lacks micro-crystalline grain boundaries, breaking down to single-molecule thickness.
Field identification & locations
Identified by its shiny glassy luster, intense black color, and characteristic curved (conchoidal) shell-like fractures. Frequently found in volcanic fields like Oregon, Mexico, and Iceland.
