igneous

Obsidian

Obsidian (principally SiO2/Silicon dioxide)

Hardness: 5-6 (Mohs); Color: Jet black to dark brownish-red; Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Fracture: Conchoidal (shell-like curves); Specific Gravity: 2.3 - 2.6.

Hardness: 5-6 (Mohs)
Color: Jet black to dark brownish-red
Luster: Vitreous/Glassy

Identified March 15, 2026More igneous →

Identify your own rocks.

Get a report just like this from any photo, free.

App Store Google Play

Physical properties

Hardness: 5-6 (Mohs); Color: Jet black to dark brownish-red; Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Fracture: Conchoidal (shell-like curves); Specific Gravity: 2.3 - 2.6.

Formation & geological history

Formed through the rapid cooling of silica-rich felsic lava, preventing crystal growth. Most specimens date from the Pliocene to the Holocene, as obsidian is geologically unstable and eventually devitrifies into minerals like quartz over millions of years.

Uses & applications

Used historically for razor-sharp tools and weapons. Currently used in jewelry (bracelets, beads), decorative carvings, and in high-precision surgical scalpel blades because it holds an edge sharper than steel.

Geological facts

Obsidian fractures down to a single atom, making its edge much sharper than a surgical steel scalpel. Native Americans used it for arrowheads and trade, with some pieces traveling thousands of miles from their source.

Field identification & locations

Identify in the field by its characteristic conchoidal fracture which leaves sharp, curved edges. Look for it in volcanic regions such as the American Northwest (Oregon/California) or Iceland. Always check for translucent edges when held to a light source.

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

Sodalite

Sodalite - Na8(Al6Si6O24)Cl2

mineral

Blue Apatite

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

mineral

Magnetite

Magnetite (Fe3O4)

mineral