igneous

Obsidian

Obsidian (Volcanic glass), SiO2 (major component)

Hardness: 5–6 (Mohs); Color: Black (jet black to smoky grey); Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Cleavage: Conchoidal fracture; Specific gravity: 2.3–2.6.

Hardness: 5–6 (Mohs)
Color: Black (jet black to smoky grey)
Luster: Vitreous/Glassy

Identified March 20, 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: Black (jet black to smoky grey); Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Cleavage: Conchoidal fracture; Specific gravity: 2.3–2.6.

Formation & geological history

Formed from extrusive igneous activity when highly felsic lava (rich in silica) cools extremely rapidly, preventing the growth of mineral crystals. Typically found in young volcanic regions (Neogene to Quaternary period) as it devitrifies/decomposes over geological time.

Uses & applications

Used historically for stone tools and weaponry (arrowheads, scalpels); currently used in jewelry (cabochons), surgical blades, and high-end ornamental décor.

Geological facts

Ancient Romans used it as mirrors. Because it breaks on the molecular level, obsidian blades are significantly sharper than stainless steel surgical scalpels. It is not technically a mineral because it lacks a crystalline structure.

Field identification & locations

Identify by its characteristic shell-like (conchoidal) fracture lines and glassy texture. Commonly found in Oregon, Mexico, Iceland, and Italy. Collectors should look for sharp edges and transparency in thin flakes.

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