igneous

Obsidian

Obsidian (Volcanic Glass), primarily SiO2

Hardness: 5–5.5 (Mohs scale); Color: Black, dark green, or brown; Luster: Vitreous (glassy); Crystal structure: Amorphous (non-crystalline); Cleavage: None (conchoidal fracture); Specific gravity: 2.35–2.6

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

Identified March 25, 2026More igneous →

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Physical properties

Formation & geological history

Formed from extrusive igneous activity when felsic lava cools so rapidly that crystals do not have time to grow. It is common in geologically recent volcanic areas, often from the Quaternary period.

Uses & applications

Used historically for sharp tools and weapons (scalpels, arrowheads). Modern uses include jewelry, decorative carvings, and as a gemstone for collectors.

Geological facts

Because it doesn't have a crystal structure, obsidian fractures into extremely sharp edges, leading to its historic use in surgery and hunting. Unlike most rocks, it is technically a 'mineraloid'.

Field identification & locations

Identify in the field by its high-gloss vitreous shine and smooth, curved (conchoidal) fracture surfaces. Commonly found in Iceland, Mexico, the United States (Oregon/California), and Japan.

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