igneous
Black Obsidian
Volcanic Glass (predominantly SiO2, Silicon Dioxide)
Hardness: 5-5.5 on Mohs scale; Color: Jet black to dark gray; Luster: Vitreous (glassy); Crystal structure: Amorphous (non-crystalline); Cleavage: None, exhibits conchoidal (shell-like) fracture; Specific Gravity: 2.35-2.60.
- Hardness
- 5-5
- Color
- Jet black to dark gray
- Luster
- Vitreous (glassy)
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Physical properties
Hardness: 5-5.5 on Mohs scale; Color: Jet black to dark gray; Luster: Vitreous (glassy); Crystal structure: Amorphous (non-crystalline); Cleavage: None, exhibits conchoidal (shell-like) fracture; Specific Gravity: 2.35-2.60.
Formation & geological history
Obsidian forms when felsic lava extruded from a volcano cools very rapidly with minimal crystal growth. It is technically an 'extrusive igneous rock' and is usually found in geologically young areas, as glass breaks down over millions of years through hydration.
Uses & applications
Used primarily in jewelry as pendants, beads, and cabochons. Historically used for extremely sharp blades, arrowheads, and mirrors. In modern medicine, it is sometimes used for scalpel blades due to its ability to be sharper than steel.
Geological facts
Because it lacks a crystal structure, obsidian is not a true mineral but a 'mineraloid.' It was one of the most important materials for trade in pre-Columbian Mesoamerica. Some varieties, like 'Rainbow Obsidian,' show iridescent sheen due to microscopic inclusions of magnetite.
Field identification & locations
Identify by its glassy texture, deep black opacity, and sharp curved edges (conchoidal fracture). It is common in volcanic regions like the American West (Oregon, California), Mexico, Iceland, and Italy. Note: Very similar looking 'Black Onyx' is a harder microcrystalline quartz (Hardness 7).
