igneous
Obsidian
Volcanic glass (SiO2 - Silicon Dioxide with impurities)
Hardness: 5-5.5 on Mohs scale; Color: typically black (but can be green, brown, or striped); Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Cleavage: None (conchoidal fracture); Specific Gravity: 2.4
- Hardness
- 5-5
- Color
- typically black (but can be green, brown, or striped)
- Luster
- Vitreous/Glassy
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Physical properties
Hardness: 5-5.5 on Mohs scale; Color: typically black (but can be green, brown, or striped); Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Cleavage: None (conchoidal fracture); Specific Gravity: 2.4
Formation & geological history
Formed through the rapid cooling of felsic lava, preventing the growth of mineral crystals. It is common in areas with recent volcanic activity, dating from the Cenozoic era back to much older periods, though it devitrifies over millions of years.
Uses & applications
Historically used for arrowheads and blades; currently used in jewelry (as a gemstone), ornamental sculptures, and in high-precision surgical scalpel blades due to its ability to hold an edge thinner than steel.
Geological facts
Obsidian fractures with extremely sharp edges. Unlike most rocks, it is technically a glass and a mineraloid rather than a mineral because it lacks a crystal structure. It was highly prized in pre-Columbian Mesoamerica for cutting tools and mirrors.
Field identification & locations
Identify in the field by its distinct conchoidal (shell-like) fracture patterns and glassy texture. Common locations include the United States (Oregon, California, Arizona), Iceland, Mexico, and Italy. Collectors look for unique varieties like 'snowflake' or 'rainbow' obsidian.
