igneous

Tumbled Obsidian (possibly Mahogany Obsidian variant)

Volcanic Glass (Amorphous Silicon Dioxide, SiO2)

Hardness: 5-6 (Mohs scale); Color: Black with potential reddish-brown (iron) inclusions; Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Cleavage: None (conchoidal fracture); Specific Gravity: 2.3–2.6

Hardness: 5-6 (Mohs scale)
Color: Black with potential reddish-brown (iron) inclusions
Luster: Vitreous/Glassy

Identified March 26, 2026More igneous →

Identify your own rocks.

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

App Store Google Play

Physical properties

Formation & geological history

Formed from the rapid cooling of silica-rich rhyolitic lava, which prevents crystal growth. This process typically occurs in volcanic margins or locations where lava contacts water or air quickly. Age varies by volcanic site but is geologically young as glass devitrifies over millions of years.

Uses & applications

Used historically for sharp tools and weapons (arrowheads, blades); modern uses include surgical scalpels, jewelry (beads, cabochons), and ornamental carvings.

Geological facts

Obsidian is technically a glass, not a mineral, because it lacks a crystalline structure. Ancient civilizations believed it held protective powers. Even today, it is used in some specialized eye surgeries because it can be honed to a width only atoms thick.

Field identification & locations

Field identification is marked by its glassy texture and razor-sharp conchoidal (shell-like) fracturing. Commonly found in areas of recent volcanic activity like the American West (Iceland, Mexico, and the Cascades). Collectors should look for its unique smooth, curved break patterns.

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