igneous
Obsidian
Obsidian (Volcanic Glass), primarily SiO2 (Silicon Dioxide)
Hardness: 5-5.5 on Mohs scale; Color: Black, deep brown, or gray; Luster: Vitreous (glassy); Crystal Structure: Amorphous (non-crystalline); Cleavage: None, exhibits conchoidal (shell-like) fracture; Specific Gravity: 2.4
- Hardness
- 5-5
- Color
- Black, deep brown, or gray
- Luster
- Vitreous (glassy)
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Physical properties
Hardness: 5-5.5 on Mohs scale; Color: Black, deep brown, or gray; Luster: Vitreous (glassy); Crystal Structure: Amorphous (non-crystalline); Cleavage: None, exhibits conchoidal (shell-like) fracture; Specific Gravity: 2.4
Formation & geological history
Formed through the rapid cooling of felsic (silica-rich) lava, which prevents crystal growth. Most specimens found today are younger than the Cretaceous period (under 66 million years) because the glass eventually devitrifies into crystalline rock.
Uses & applications
Used historically for making sharp tools like arrowheads and knives. Modern uses include surgical scalpel blades, jewelry (pendants and beads), and decorative ornaments.
Geological facts
Obsidian is technically a 'mineraloid' rather than a mineral because it lacks a crystalline structure. It was highly prized in ancient Mesoamerican cultures for both utility and ritual purposes, such as Aztec sacrificial knives.
Field identification & locations
Identify in the field by its distinctive glassy texture and sharp, curved edges when broken. Common locations include the Western United States (Oregon, California, Arizona), Iceland, Mexico, Italy, and Japan.
