igneous
Obsidian
Obsidian (Volcanic Glass), primarily SiO2 (Silicon Dioxide)
Hardness: 5-5.5 on Mohs scale; Color: typically black but can show sheen or translucency; Luster: Vitreous (glassy); Crystal Structure: Amorphous (lacks crystal structure); Cleavage: Conchoidal fracture
- Hardness
- 5-5
- Color
- typically black but can show sheen or translucency
- Luster
- Vitreous (glassy)
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Physical properties
Hardness: 5-5.5 on Mohs scale; Color: typically black but can show sheen or translucency; Luster: Vitreous (glassy); Crystal Structure: Amorphous (lacks crystal structure); Cleavage: Conchoidal fracture
Formation & geological history
Formed through the extremely rapid cooling of felsic lava, which prevents the growth of crystals. It is found in geologically young areas with recent volcanic activity, predominantly from the Cenozoic era.
Uses & applications
Used historically for sharp tools and weapons (scalpels, arrowheads). In modern times, it is used for jewelry, decorative carvings, gemstones, and as a high-quality material for surgical blades.
Geological facts
Because it has no crystal structure, obsidian can be honed to an edge only a few molecules thick, making it sharper than even the finest steel surgical blades. It is also known as 'Dragon Glass' in popular culture (Game of Thrones).
Field identification & locations
Identify in the field by its distinctive glassy appearance and shell-like (conchoidal) fractures. Common locations include the Western USA (Oregon, California), Mexico, Iceland, and Italy. Collectors look for specific optical effects like 'sheen' or 'snowflake' patterns.
