igneous

Obsidian

Volcanic Glass (SiO2 rich)

Hardness: 5-5.5 (Mohs scale); Color: Black, sometimes grayish or brown; Luster: Vitreous (glassy); Crystal structure: Amorphous (non-crystalline); Cleavage: Conchoidal fracture; Specific gravity: 2.4

Hardness: 5-5
Color: Black, sometimes grayish or brown
Luster: Vitreous (glassy)

Identified April 2, 2026More igneous →

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Physical properties

Hardness: 5-5.5 (Mohs scale); Color: Black, sometimes grayish or brown; Luster: Vitreous (glassy); Crystal structure: Amorphous (non-crystalline); Cleavage: Conchoidal fracture; Specific gravity: 2.4

Formation & geological history

Formed from rapid cooling of felsic lava, preventing crystal growth. Usually found in geologically young volcanic areas (Cenozoic) because the glass is unstable over long geological time and eventually devitrifies.

Uses & applications

Used in surgical scalpel blades for extreme sharpness, gemstone jewelry, ornamental sculptures, and historically for stone tools like arrowheads and knives.

Geological facts

Obsidian is often called 'nature's glass'. Because it lacks a crystal structure, its edges can be thin enough to be molecularly sharp, significantly sharper than high-quality steel scalpels.

Field identification & locations

Identify in the field by its distinct glassy luster and conchoidal 'shell-like' fractures. Commonly found in locations like Yellowstone (USA), Iceland, Mexico, and Japan. Best identified by its sharp edges and lack of visible grains.

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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