igneous

Black Obsidian

Volcanic Glass (predominantly SiO2, Silicon Dioxide)

Hardness: 5-5.5 on Mohs scale; Color: Jet black to dark gray; Luster: Vitreous (glassy); Crystal structure: Amorphous (non-crystalline); Cleavage: None, exhibits conchoidal (shell-like) fracture; Specific Gravity: 2.35-2.60.

Hardness: 5-5
Color: Jet black to dark gray
Luster: Vitreous (glassy)

Identified March 28, 2026More igneous →

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Physical properties

Formation & geological history

Obsidian forms when felsic lava extruded from a volcano cools very rapidly with minimal crystal growth. It is technically an 'extrusive igneous rock' and is usually found in geologically young areas, as glass breaks down over millions of years through hydration.

Uses & applications

Used primarily in jewelry as pendants, beads, and cabochons. Historically used for extremely sharp blades, arrowheads, and mirrors. In modern medicine, it is sometimes used for scalpel blades due to its ability to be sharper than steel.

Geological facts

Because it lacks a crystal structure, obsidian is not a true mineral but a 'mineraloid.' It was one of the most important materials for trade in pre-Columbian Mesoamerica. Some varieties, like 'Rainbow Obsidian,' show iridescent sheen due to microscopic inclusions of magnetite.

Field identification & locations

Identify by its glassy texture, deep black opacity, and sharp curved edges (conchoidal fracture). It is common in volcanic regions like the American West (Oregon, California), Mexico, Iceland, and Italy. Note: Very similar looking 'Black Onyx' is a harder microcrystalline quartz (Hardness 7).

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