igneous

Obsidian

Volcanic Glass (Amorphous SiO2)

Hardness (Mohs scale): 5.0–6.0, Color: Black/dark brown, Luster: Vitreous/glassy, Crystal structure: Amorphous (non-crystalline), Cleavage: None (conchoidal fracture), Specific gravity: 2.35–2.6

Identified May 21, 2026More igneous →

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Physical properties

Hardness (Mohs scale): 5.0–6.0, Color: Black/dark brown, Luster: Vitreous/glassy, Crystal structure: Amorphous (non-crystalline), Cleavage: None (conchoidal fracture), Specific gravity: 2.35–2.6

Formation & geological history

Formed through the rapid cooling of silica-rich (felsic) lava, preventing crystal growth. Usually found in margin of lava flows or where lava contacts water. Most specimens are younger than Cretaceous age as glass eventually devitrifies.

Uses & applications

Used historically for sharp tools and weapons (arrowheads/knives); currently used for jewelry, surgical scalpel blades, and ornamental purposes.

Geological facts

Unlike almost all other rocks, obsidian is not made of minerals; it is a natural glass. It is often sharper than high-quality steel surgical scalpels because its edge can reach molecular thinness.

Field identification & locations

Identify by its characteristic conchoidal fracture (curved, shell-like ripples) and glassy luster. Commonly found in volcanic regions like the American West, Iceland, Italy, and Mexico.

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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