igneous
Obsidian
Volcanic Glass (predominantly SiO2)
Hardness: 5-5.5 on Mohs scale; Color: typically jet black, can be green, brown, or grayish; Luster: Vitreous (glassy); Crystal Structure: Amorphous (non-crystalline); Fracture: Conchoidal (shell-like curves); Specific Gravity: 2.4
- Hardness
- 5-5
- Color
- typically jet black, can be green, brown, or grayish
- Luster
- Vitreous (glassy)
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Physical properties
Hardness: 5-5.5 on Mohs scale; Color: typically jet black, can be green, brown, or grayish; Luster: Vitreous (glassy); Crystal Structure: Amorphous (non-crystalline); Fracture: Conchoidal (shell-like curves); Specific Gravity: 2.4
Formation & geological history
Formed when felsic lava extruded from a volcano cools rapidly with minimal crystal growth. It is found in geologically recent volcanic areas, often from the Pliocene to the Holocene, as glass degrades over time into minerals.
Uses & applications
Historically used for edged tools (scalpels, arrowheads) due to its razor-sharp fracture. Today used in jewelry (beads, cabochons), ornamental mirrors, and high-end surgical blades.
Geological facts
Because it is a glass rather than a crystal, obsidian fractures down to a molecular thickness, making it sharper than even the highest quality steel scalpels. It was highly prized by ancient civilizations like the Mayans and Aztecs for ritual weaponry.
Field identification & locations
Identify in the field by its distinct vitreous luster and smooth, curved conchoidal fracture patterns. Commonly found in Iceland, Mexico, Italy, and the Western United States (Oregon, California, Arizona). Collectors look for variations like 'Mahogany', 'Rainbow', or 'Snowflake' obsidian.
