igneous
Obsidian
Volcanic glass (mainly SiO2 with inclusions)
Hardness: 5–5.5 on Mohs scale; Color: Black, deep grey, or dark brown; Luster: Vitreous (glassy); Crystal Structure: Amorphous (non-crystalline); Cleavage: None (conchoidal fracture); Specific Gravity: 2.35–2.6.
- Hardness
- 5–5
- Color
- Black, deep grey, or dark brown
- Luster
- Vitreous (glassy)
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Physical properties
Hardness: 5–5.5 on Mohs scale; Color: Black, deep grey, or dark brown; Luster: Vitreous (glassy); Crystal Structure: Amorphous (non-crystalline); Cleavage: None (conchoidal fracture); Specific Gravity: 2.35–2.6.
Formation & geological history
Formed from extrusive igneous activity when felsic lava cools rapidly with minimal crystal growth. Common in rhyolitic volcanic eruptions spanning from the Cenozoic era to recent times.
Uses & applications
Historically used for spearheads and blades; currently used in jewelry (cabochons/faceting), ornamental carvings, and high-precision surgical scalpels.
Geological facts
Because it is essentially a supercooled liquid, obsidian is metastable and eventually devitrifies into quartz and feldspar over millions of years. It lacks the crystalline regularity of minerals, making it a mineraloid.
Field identification & locations
Identify in the field by its sharp, curved (conchoidal) edges and glass-like appearance. Commonly found in volcanic regions such as the American Northwest (Oregon, California), Mexico, Iceland, and Italy (Lipari).
