Igneous

Obsidian

Volcanic glass (mainly SiO2)

Hardness: 5-5.5 (Mohs); Color: Jet black, sometimes gray or brown; Luster: Vitreous (glassy); Structure: Amorphous (non-crystalline); Cleavage: Conchoidal fracture; Specific gravity: 2.4

Hardness: 5-5
Color: Jet black, sometimes gray or brown
Luster: Vitreous (glassy)

Identified March 25, 2026More igneous →

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Physical properties

Hardness: 5-5.5 (Mohs); Color: Jet black, sometimes gray or brown; Luster: Vitreous (glassy); Structure: Amorphous (non-crystalline); Cleavage: Conchoidal fracture; Specific gravity: 2.4

Formation & geological history

Formed through the rapid cooling of felsic (silicic) lava, which prevents the growth of crystals. It is most common in geologically recent volcanic areas (Cenozoic era) as it eventually devitrifies into crystalline rock over millions of years.

Uses & applications

Used for making surgical scalpels due to its ability to hold an edge thinner than steel, ornamental jewelry (beads, cabochons), and collectible decorative pieces. Historically used for weapons like arrowheads and knives.

Geological facts

Because it's a glass, obsidian lacks an internal crystal structure. Ancient Mesoamerican civilizations used it both for lethal weaponry and as ritual mirrors. The edges of a freshly broken obsidian shard can be only a few molecules thick.

Field identification & locations

Identify in the field by its distinct shell-like (conchoidal) fracture patterns, sharp translucent edges, and glassy appearance. Commonly found in Oregon (Glass Buttes), Mexico, Iceland, and the Lipari Islands.

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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