igneous
Obsidian
Obsidian (Volcanic Glass; SiO2 content > 70%)
Hardness: 5–6 Mohs; Color: Jet black (though can be green, brown, or gray in thin sections); Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Cleavage: None (exhibits conchoidal fracture); Specific Gravity: 2.35–2.6
- Hardness
- 5–6 Mohs
- Luster
- Vitreous/Glassy
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Physical properties
Hardness: 5–6 Mohs; Color: Jet black (though can be green, brown, or gray in thin sections); Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Cleavage: None (exhibits conchoidal fracture); Specific Gravity: 2.35–2.6
Formation & geological history
Formed from the rapid cooling of silica-rich (felsic) lava, prevents crystal growth. Usually found in volcanic regions of any geological age, though most specimens are relatively young because obsidian eventually devitrifies into crystalline rock.
Uses & applications
Used in ornamental jewelry, as a healing stone in lapidary, and in specialized surgical scalpels due to its ability to hold an edge sharper than steel. Historically used for weapons like arrowheads and knives.
Geological facts
Because it lacks a crystal structure, obsidian is technically a 'mineraloid' rather than a mineral. Native Americans traded it extensively across North America, and it was the material used for Aztec 'Macuahuitl' swords.
Field identification & locations
Identify in the field by its distinct conchoidal (shell-like) fracture, sharp edges, and glassy appearance. Common locations include the Western USA (Oregon, California), Iceland, Mexico, and Japan. Collectors look for translucency at the edges.
