igneous
Obsidian
Obsidian (principally SiO2/Silicon dioxide)
Hardness: 5-6 (Mohs); Color: Jet black to dark brownish-red; Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Fracture: Conchoidal (shell-like curves); Specific Gravity: 2.3 - 2.6.
- Hardness
- 5-6 (Mohs)
- Color
- Jet black to dark brownish-red
- Luster
- Vitreous/Glassy
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Physical properties
Hardness: 5-6 (Mohs); Color: Jet black to dark brownish-red; Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Fracture: Conchoidal (shell-like curves); Specific Gravity: 2.3 - 2.6.
Formation & geological history
Formed through the rapid cooling of silica-rich felsic lava, preventing crystal growth. Most specimens date from the Pliocene to the Holocene, as obsidian is geologically unstable and eventually devitrifies into minerals like quartz over millions of years.
Uses & applications
Used historically for razor-sharp tools and weapons. Currently used in jewelry (bracelets, beads), decorative carvings, and in high-precision surgical scalpel blades because it holds an edge sharper than steel.
Geological facts
Obsidian fractures down to a single atom, making its edge much sharper than a surgical steel scalpel. Native Americans used it for arrowheads and trade, with some pieces traveling thousands of miles from their source.
Field identification & locations
Identify in the field by its characteristic conchoidal fracture which leaves sharp, curved edges. Look for it in volcanic regions such as the American Northwest (Oregon/California) or Iceland. Always check for translucent edges when held to a light source.
