igneous
Obsidian
Obsidian (Volcanic Glass, SiO2 > 70%)
Hardness: 5-5.5 (Mohs scale); Color: Black, dark green, or brownish; Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Cleavage: None (conchoidal fracture); Specific Gravity: 2.3-2.6.
- Hardness
- 5-5
- Color
- Black, dark green, or brownish
- Luster
- Vitreous/Glassy
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Physical properties
Hardness: 5-5.5 (Mohs scale); Color: Black, dark green, or brownish; Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Cleavage: None (conchoidal fracture); Specific Gravity: 2.3-2.6.
Formation & geological history
Formed through the rapid cooling of silica-rich (felsic) lava, which prevents crystal growth. It is typically found in geologically young volcanic regions (Quaternary to Tertiary ages) because it is unstable and eventually devitrifies.
Uses & applications
Used in the manufacture of surgical scalpel blades, jewelry, decorative ornaments, and historically for sharp tools and weapons like arrowheads and knives.
Geological facts
Obsidian is technically a glass rather than a true mineral because it lacks a crystalline structure. Ancient Aztecs and Mayans used it as mirrors and sacred objects. Its conchoidal fracture creates edges thinner and sharper than the finest steel scalpels.
Field identification & locations
Identify in the field by its distinct glassy appearance, lack of grain, and sharp, shell-like (conchoidal) curved fractures. Common locations include Oregon, California, Iceland, Mexico, and Japan.
