igneous

Obsidian

Obsidian (principally SiO2 + MgO, Fe3O4)

Hardness: 5-5.5 on Mohs scale; Color: Black, sometimes with gray or brown; Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Cleavage: None (conchoidal fracture); Specific Gravity: 2.4

Hardness: 5-5
Color: Black, sometimes with gray or brown
Luster: Vitreous/Glassy

Identified March 28, 2026More igneous →

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Physical properties

Hardness: 5-5.5 on Mohs scale; Color: Black, sometimes with gray or brown; Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Cleavage: None (conchoidal fracture); Specific Gravity: 2.4

Formation & geological history

Formed from the rapid cooling of silica-rich (felsic) lava, which prevents crystal growth. Most specimens observed are from the Cenozoic era or younger as it devitrifies over time.

Uses & applications

Used historically for sharp tools and weapons (scalpels, arrowheads). Modern uses include jewelry, decorative carvings, and specialized surgical blades.

Geological facts

Obsidian is technically a natural volcanic glass rather than a true mineral because it lacks a crystalline structure. It was highly prized in the Stone Age for its razor-sharp edges.

Field identification & locations

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

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