igneous
Obsidian
Obsidian (Volcanic Glass)
Hardness: 5-5.5 (Mohs scale); Color: Black, sometimes with brown or grey; Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Cleavage: Conchoidal fracture; Specific gravity: 2.3-2.6
- Hardness
- 5-5
- Color
- Black, sometimes with brown or grey
- Luster
- Vitreous/Glassy
Identified More igneous →
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Physical properties
Hardness: 5-5.5 (Mohs scale); Color: Black, sometimes with brown or grey; Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Cleavage: Conchoidal fracture; Specific gravity: 2.3-2.6
Formation & geological history
Formed from extrusive igneous activity when silica-rich felsic lava cools so rapidly that crystals do not have time to grow. Usually associated with rhyolitic volcanic eruptions from the Cenozoic era.
Uses & applications
Used for surgical scalpels, jewelry (cabochons and beads), gemstones, and ornamental carvings. Historically used for arrowheads and cutting tools.
Geological facts
Obsidian is not a true mineral because it lacks a crystal structure. It is often referred to as 'nature's glass'. Famous varieties include Snowflake Obsidian and Rainbow Obsidian.
Field identification & locations
Identify by its extremely smooth, curved (conchoidal) fracture surfaces and glassy luster. Found in areas of recent volcanic activity such as the Cascades, Iceland, Mexico, and Japan.
