igneous
Obsidian
Volcanic Glass (70-75% SiO2 with MgO and Fe3O4)
Hardness: 5.0-6.0 (Mohs scale). Color: Typically jet black, but can be green, brown, or red. Luster: Vitreous/Glassy. Crystal Structure: Amorphous (non-crystalline). Cleavage: None (conchoidal fracture). Specific Gravity: 2.35-2.6.
- Hardness
- 5
- Color
- Typically jet black, but can be green, brown, or red
- Luster
- Vitreous/Glassy
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Physical properties
Hardness: 5.0-6.0 (Mohs scale). Color: Typically jet black, but can be green, brown, or red. Luster: Vitreous/Glassy. Crystal 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 (extrusive igneous process) which prevents mineral growth. Generally found in geologically young areas where volcanic activity has occurred, typically less than 20 million years old as it eventually devitrifies.
Uses & applications
Used primarily in jewelry (beads and cabochons), ornamental sculpture, and high-end surgical scalpel blades due to its ability to produce an edge thinner than steel. Historically vital for tools like arrowheads and knives.
Geological facts
Because it is a glass and lacks a crystalline structure, obsidian is technically a mineraloid rather than a mineral. It was widely traded by ancient civilizations as a precious material for weapon-making and ritual objects.
Field identification & locations
Identify by its deep black color, high gloss, and distinctive shell-like (conchoidal) fractures with extremely sharp edges. Commonly found in volcanic regions like Iceland, Mexico, Italy, and the western United States (e.g., Oregon and California).
