igneous
Black Obsidian
Obsidian (Volcanic Glass), primarily SiO2 (Silicon Dioxide)
Hardness: 5–5.5 Mohs scale; Color: Jet black, often translucent at thin edges; Luster: Vitreous (glassy); Crystal Structure: Amorphous (non-crystalline); Cleavage: None (conchoidal fracture); Specific Gravity: 2.4
- Hardness
- 5–5
- Color
- Jet black, often translucent at thin edges
- Luster
- Vitreous (glassy)
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Physical properties
Hardness: 5–5.5 Mohs scale; Color: Jet black, often translucent at thin edges; Luster: Vitreous (glassy); Crystal Structure: Amorphous (non-crystalline); Cleavage: None (conchoidal fracture); Specific Gravity: 2.4
Formation & geological history
Formed from the rapid cooling of felsic (silica-rich) lava, which prevents the growth of crystals. It is found in volcanic regions where cooling occurred quickly, often near the edges of lava flows or where lava hit water.
Uses & applications
Commonly used in jewelry as cabochons or pendants, for ornamental carvings, and in 'crystal healing' practices. Historically used for making razor-sharp scalpels, arrowheads, and blades due to its conchoidal fracturing.
Geological facts
Obsidian can produce a cutting edge that is many times sharper than high-quality steel surgical scalpels because it breaks at a molecular level. It is technically a mineraloid rather than a true mineral because it lacks a crystalline structure.
Field identification & locations
Identify in the field by its distinct glassy luster and smooth, shell-like (conchoidal) fracture patterns. Frequently found in regions with recent volcanic activity like Oregon (USA), Iceland, Japan, and Mexico. Collectors look for uniform color or unique patterns like 'snowflake' inclusions.
