igneous
Obsidian
Obsidian (principally SiO2 + MgO, Fe3O4)
Hardness: 5-5.5 on Mohs scale. Color: Jet black to dark grey. Luster: Vitreous (glassy). Structure: Amorphous (non-crystalline). Cleavage: None (conchoidal fracture). Specific Gravity: 2.4.
- Hardness
- 5-5
- Color
- Jet black to dark grey
- Luster
- Vitreous (glassy)
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Physical properties
Hardness: 5-5.5 on Mohs scale. Color: Jet black to dark grey. Luster: Vitreous (glassy). Structure: Amorphous (non-crystalline). Cleavage: None (conchoidal fracture). Specific Gravity: 2.4.
Formation & geological history
Formed from the rapid cooling of silica-rich (felsic) lava, which prevents the growth of crystals. It is geologically young, typically found in areas with Cenozoic volcanic activity, as it eventually devitrifies into crystalline rock over millions of years.
Uses & applications
Historically used for spearheads and knives due to its razor-sharp edges. Modernly used in high-end surgical scalpels, gemstone jewelry (beads, cabochons), and ornamental carvings.
Geological facts
Obsidian is often referred to as 'Nature's Glass.' Because it has no crystal structure, its edges can be thinned down to a molecular level, making it sharper than even high-quality steel surgical scalpels.
Field identification & locations
Identify in the field by its distinct conchoidal (shell-like) fracture patterns and extremely smooth, glassy surface. Commonly found in volcanic regions such as the American Northwest (Oregon, California), Iceland, Japan, and Mexico.
