igneous
Obsidian (in jewelry setting)
Volcanic Glass (Amorphous Silica)
Hardness: 5-5.5 (Mohs scale); Color: Typically deep black (due to iron and magnesium); Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Cleavage: None (conchoidal fracture); Specific Gravity: 2.3 - 2.6.
- Hardness
- 5-5
- Color
- Typically deep black (due to iron and magnesium)
- Luster
- Vitreous/Glassy
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Physical properties
Hardness: 5-5.5 (Mohs scale); Color: Typically deep black (due to iron and magnesium); Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Cleavage: None (conchoidal fracture); Specific Gravity: 2.3 - 2.6.
Formation & geological history
Formed through the rapid cooling of felsic lava, which prevents the growth of mineral crystals. It is common in volcanically active regions and is generally geologically young (less than 65 million years) as glass eventually devitrifies.
Uses & applications
Used primarily for jewelry, ornamental objects, and surgical scalpel blades (due to its ability to be honed to an edge much sharper than steel). It is also highly valued by gemstone collectors and for decorative metaphysical items.
Geological facts
Because it breaks with a conchoidal fracture, it creates edges that are molecularly thin. In the Stone Age, it was the primary material for high-quality arrowheads and blades. It shares the same chemical composition as granite but cooled too fast to crystallize.
Field identification & locations
Identify in the field by its characteristic glassy texture and curved, shell-like (conchoidal) fractures. Found in volcanic areas with silica-rich lava such as the Western USA, Mexico, Iceland, and Japan. Collectors look for unique variations like 'snowflake' or 'rainbow' obsidian.
