igneous
Obsidian
Volcanic Glass (SiO2)
Hardness: 5-5.5 Mohs; Color: Black to dark brown; Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Cleavage: Conchoidal fracture; SG: 2.4
- Hardness
- 5-5
- Color
- Black to dark brown
- Luster
- Vitreous/Glassy
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Physical properties
Hardness: 5-5.5 Mohs; Color: Black to dark brown; Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Cleavage: Conchoidal fracture; SG: 2.4
Formation & geological history
Formed from extrusive igneous activity when highly felsic lava cools rapidly, preventing crystal growth. Common in volcanic regions across central Mexico (Trans-Mexican Volcanic Belt).
Uses & applications
Used historically for sharp tools and weaponry; modern use in surgical scalpel blades, jewelry, and ornamental carvings.
Geological facts
Obsidian is naturally occurring glass; it is chemically similar to granite but cooled too fast to form minerals. The location 20.0°N, 100.1°W is near Queretaro/Hidalgo, Mexico, a region famous for significant obsidian deposits used by pre-Hispanic civilizations.
Field identification & locations
Identify by its glass-like texture and sharp, curving conchoidal fracture edges. Very common in the volcanic highlands of Mexico.
