igneous

Obsidian

Volcanic Glass (predominantly SiO2)

Hardness: 5-5.5 (Mohs); Color: Black to dark brown/green; Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Cleavage: Conchoidal fracture; Specific Gravity: 2.4

Hardness: 5-5
Color: Black to dark brown/green
Luster: Vitreous/Glassy

Identified March 14, 2026More igneous →

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Physical properties

Hardness: 5-5.5 (Mohs); Color: Black to dark brown/green; Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Cleavage: Conchoidal fracture; Specific Gravity: 2.4

Formation & geological history

Formed when extrusive felsic lava cools rapidly with minimal crystal growth. Typically found in volcanic areas from the Cenozoic era onwards (as it devitrifies over time).

Uses & applications

Historically used for arrowheads and blades; currently used in scalpel manufacturing, jewelry (cabs/beads), and as an ornamental stone.

Geological facts

Because it is glass and not a crystal, it breaks into edges that are thinner and sharper than a high-quality steel scalpel. Ancient Aztecs used it for sacrificial knives and mirrors.

Field identification & locations

Identify by its characteristic conchoidal (curved, shell-like) fracture and smooth, glassy texture. Found globally in volcanic regions like the American West (Glass Buttes), Iceland, and Mexico.

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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