igneous

Obsidian

Obsidian (Volcanic Glass)

Hardness: 5–6 on the Mohs scale; Color: typically jet black, can be grey, green, or brown; Luster: Vitreous (glassy); Crystal structure: Amorphous (none); Cleavage: None (conchoidal fracture); Specific gravity: 2.35–2.6.

Hardness: 5–6 on the Mohs scale
Color: typically jet black, can be grey, green, or brown
Luster: Vitreous (glassy)

Identified February 8, 2026More igneous →

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Physical properties

Formation & geological history

Obsidian is produced when felsic lava extruded from a volcano cools rapidly with minimal crystal growth. It is found in volcanic regions where the cooling occurred quickly, often at the edges of a lava flow or where lava contacted water or air.

Uses & applications

Historically used for making sharp tools like arrowheads and blades; currently used in jewelry (gemstones), ornamental carvings, and specialized surgical scalpels due to its incredibly sharp edge.

Geological facts

Because it lacks a crystalline structure, obsidian edges can reach nearly molecular thinness, making them sharper than steel scalpels. It was highly prized by ancient civilizations like the Aztecs and Mayans.

Field identification & locations

Identify in the field by its distinct conchoidal (shell-like) fracture patterns and glassy luster. It is common in areas with recent volcanic activity like the American Northwest, Iceland, Mexico, and Japan.

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Blue Apatite

Fluorapatite (Ca5(PO4)3F)

mineral

Dark Igneous Rock (Likely Basalt or Diabase)

Basalt (extrusive igneous rock) or Diabase (intrusive igneous rock - also known as Dolerite), largely composed of plagioclase feldspar, pyroxene (augite), and sometimes olivine and amphibole. Exact mineralogy would require thin section analysis.

Igneous