Igneous
Obsidian
Volcanic Glass (Primary composition: SiO2 - Silicon Dioxide)
Hardness: 5-6 Mohs scale; Color: Jet black (can be dark green or brown); Luster: Vitreous/glassy; Structure: Amorphous (non-crystalline); Cleavage: None (conchoidal fracture); Specific Gravity: 2.35-2.6
- Hardness
- 5-6 Mohs scale
- Color
- Jet black (can be dark green or brown)
- Luster
- Vitreous/glassy
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Physical properties
Hardness: 5-6 Mohs scale; Color: Jet black (can be dark green or brown); Luster: Vitreous/glassy; Structure: Amorphous (non-crystalline); Cleavage: None (conchoidal fracture); Specific Gravity: 2.35-2.6
Formation & geological history
Formed through the rapid cooling of silica-rich felsic lava, which prevents the growth of mineral crystals. Typically found in volcanic areas with relatively recent eruptions (Cenozoic era).
Uses & applications
Used for surgical scalpel blades (sharper than steel), ornamental jewelry (cabochons), interior decor, and as a popular semi-precious stone for collectors and metaphysical uses.
Geological facts
Because it doesn't have a crystal structure, obsidian edges can reach molecular thinness. In pre-Columbian Mesoamerica, it was used to create the 'Macuahuitl' (a wooden sword with obsidian blades).
Field identification & locations
Identify in the field by its distinctive 'conchoidal' (shell-like) fracture patterns and sharp edges. Common in regions like the Western United States (Oregon, California), Mexico, and Iceland. Collectors should look for unique variants like 'Snowflake' or 'Rainbow' obsidian.
