igneous
Obsidian
Volcanic Glass (SiO2 with impurities)
Hardness: 5-5.5 on Mohs scale; Color: typically black but can be brown or green; Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Cleavage: None, exhibits conchoidal fracture; Specific Gravity: 2.4
- Hardness
- 5-5
- Color
- typically black but can be brown or green
- Luster
- Vitreous/Glassy
Identified More igneous →
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Physical properties
Hardness: 5-5.5 on Mohs scale; Color: typically black but can be brown or green; Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Cleavage: None, exhibits conchoidal fracture; Specific Gravity: 2.4
Formation & geological history
Formed through the rapid cooling of silica-rich felsic lava, which prevents crystal growth. It is relatively young geologically, rarely older than Cretaceous, as it tends to devitrify over time into quartz and feldspar.
Uses & applications
Used historically for sharp tools and weapons like arrowheads. Modern uses include surgical scalpel blades, jewelry, decorative carvings, and as a collector's gemstone.
Geological facts
Obsidian is technically not a mineral because it is amorphous and lacks a crystal structure. Famous varieties include Snowflake Obsidian (with spherulite inclusions) and Rainbow Obsidian.
Field identification & locations
Identify in the field by its distinctive glassy appearance and razor-sharp conchoidal (shell-like) fracture edges. Found in volcanic regions like the Cascades in the US, Iceland, and Italy.
