igneous

Obsidian

Volcanic glass (high silica SiO2)

Hardness: 5–5.5 Mohs scale; Color: Black, sometimes with dark green or brown; Luster: Vitreous (glassy); Crystal Structure: Amorphous (non-crystalline); Fracture: Conchoidal (shell-like curves); Specific Gravity: 2.4

Hardness: 5–5
Color: Black, sometimes with dark green or brown
Luster: Vitreous (glassy)

Identified March 28, 2026More igneous →

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Physical properties

Formation & geological history

Formed from extrusive igneous activity when high-silica rhyolitic lava cools rapidly, preventing crystal growth. It is found in geologically young volcanic areas usually dating from the Pliocene to the Holocene.

Uses & applications

Historically used for arrowheads and blades; currently used in high-end surgical scalpels, lapidary art, jewelry, and ornamental decor.

Geological facts

Because it is an amorphous glass and lacks crystalline structure, obsidian is not technically a mineral. It was highly prized in Mesoamerican cultures for making 'Macuahuitl' (wooden swords with obsidian blades).

Field identification & locations

Identify by its characteristic glassy surface and smooth, curved conchoidal fractures. Common in volcanic regions such as the American Northwest (Oregon/California), Mexico, Iceland, and Italy (Lipari).

Other igneous specimens

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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