igneous

Obsidian

Volcanic Glass (Amorphous Silicon Dioxide)

Hardness: 5-5.5 (Mohs); Color: Deep black to smoky brown; Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Cleavage: Conchoidal fracture; Specific gravity: 2.4

Hardness: 5-5
Color: Deep black to smoky brown
Luster: Vitreous/Glassy

Identified April 10, 2026More igneous →

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Physical properties

Hardness: 5-5.5 (Mohs); Color: Deep black to smoky brown; Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Cleavage: Conchoidal fracture; Specific gravity: 2.4

Formation & geological history

Formed from the rapid cooling of silica-rich (felsic) lava. The cooling occurs so quickly that crystals do not have time to grow, resulting in a natural glass. This specimen likely formed during volcanic eruptions in the Tertiary or Quaternary periods.

Uses & applications

Historically used for cutting tools and arrowheads; currently used in specialized surgical scalpel blades, gemstone jewelry, and decorative carvings.

Geological facts

Because it is unstable over long geological time, obsidian eventually 'devitrifies' (turns into crystalline rock), which is why no obsidian is older than the Cretaceous era. It results in some of the sharpest edges known to man.

Field identification & locations

Identify in the field by its distinct conchoidal (shell-like) fracture, sharp edges, and uniform glassy texture. It is commonly found in areas of recent volcanic activity such as Iceland, the American Northwest (Oregon/California), and Mexico.

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