igneous
Obsidian
Obsidian (principally SiO2 - Silicon Dioxide)
Hardness: 5-5.5 (Mohs scale); Color: Typically jet black, but can be green, brown, or flashy (sheen/rainbow); Luster: Vitreous (glassy); Crystal structure: Amorphous (non-crystalline); Cleavage: None (conchoidal fracture); Specific gravity: 2.4
- Hardness
- 5-5
- Luster
- Vitreous (glassy)
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Physical properties
Hardness: 5-5.5 (Mohs scale); Color: Typically jet black, but can be green, brown, or flashy (sheen/rainbow); Luster: Vitreous (glassy); Crystal structure: Amorphous (non-crystalline); Cleavage: None (conchoidal fracture); Specific gravity: 2.4
Formation & geological history
Formed through the rapid cooling of silica-rich felsic lava, which prevents the growth of mineral crystals. It is found in geologically young areas of volcanic activity, mostly from the Pliocene to the Holocene epochs.
Uses & applications
Used in jewelry (beads and cabochons), ornamental carvings, and high-end surgical scalpel blades because it can be fractured to a molecularly thin edge. Historically used for arrowheads and scrapers.
Geological facts
Obsidian is technically a natural volcanic glass rather than a mineral because it lacks a crystalline structure. Native Americans famously used obsidian from the Glass Buttes for tools and weapons.
Field identification & locations
Identify in the field by its distinct conchoidal (shell-like) fracture and sharp edges. Common locations include Oregon, California, and Iceland. Collectors look for unique varieties like 'Snowflake' or 'Rainbow' obsidian.
