igneous
Obsidian
Volcanic glass
Hardness: 5 to 5.5 on the Mohs scale, Color: Typically jet black, but can be brown, tan, or green; Luster: Vitreous (glassy); Crystal structure: Amorphous (no true crystal structure due to rapid cooling); Cleavage: Conchoidal fracture.
- Hardness
- 5 to 5
- Color
- Typically jet black, but can be brown, tan, or green
- Luster
- Vitreous (glassy)
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Physical properties
Hardness: 5 to 5.5 on the Mohs scale, Color: Typically jet black, but can be brown, tan, or green; Luster: Vitreous (glassy); Crystal structure: Amorphous (no true crystal structure due to rapid cooling); Cleavage: Conchoidal fracture.
Formation & geological history
Formed as an extrusive igneous rock when felsic lava extruded from a volcanic vent cools rapidly with minimal crystal growth. Geological age varies widely depending on the volcanic deposit.
Uses & applications
Historically used to manufacture cutting tools, arrowheads, and blades due to its exceptionally sharp conchoidal fracture. Today, it is used for ornamental objects, jewelry, and occasionally in surgical scalpel blades.
Geological facts
Because obsidian cools so quickly, it forms a natural glass rather than a mineral. Apache tears are small rounded pebbles of obsidian often found embedded in perlite.
Field identification & locations
Easily identified by its dark, glassy appearance and characteristic conchoidal (shell-like) fracture edges. Found worldwide in areas with past volcanic activity, such as the western United States, Mexico, Italy, and Japan.
