igneous

Obsidian

Obsidian (Volcanic Glass), primarily SiO2 (Silicon Dioxide)

Hardness: 5-5.5 on Mohs scale; Color: Black, deep brown, or gray; Luster: Vitreous (glassy); Crystal Structure: Amorphous (non-crystalline); Cleavage: None, exhibits conchoidal (shell-like) fracture; Specific Gravity: 2.4

Hardness: 5-5
Color: Black, deep brown, or gray
Luster: Vitreous (glassy)

Identified March 25, 2026More igneous →

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Physical properties

Formation & geological history

Formed through the rapid cooling of felsic (silica-rich) lava, which prevents crystal growth. Most specimens found today are younger than the Cretaceous period (under 66 million years) because the glass eventually devitrifies into crystalline rock.

Uses & applications

Used historically for making sharp tools like arrowheads and knives. Modern uses include surgical scalpel blades, jewelry (pendants and beads), and decorative ornaments.

Geological facts

Obsidian is technically a 'mineraloid' rather than a mineral because it lacks a crystalline structure. It was highly prized in ancient Mesoamerican cultures for both utility and ritual purposes, such as Aztec sacrificial knives.

Field identification & locations

Identify in the field by its distinctive glassy texture and sharp, curved edges when broken. Common locations include the Western United States (Oregon, California, Arizona), Iceland, Mexico, Italy, and Japan.

Other igneous specimens

See all

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

Magnetite

Iron(II,III) oxide (Fe3O4)

mineral

Blue Apatite

Apatite (Group), Ca5(PO4)3(F,Cl,OH)

mineral

Sodalite

Sodalite - Na8(Al6Si6O24)Cl2

mineral

Magnetite

Magnetite (Fe3O4)

mineral