igneous

Obsidian

Volcanic glass (SiO2 - Silicon Dioxide with impurities)

Hardness: 5-5.5 (Mohs); Color: Typically black, can be grey, green, or mahogany; Luster: Vitreous (glassy); Crystal Structure: Amorphous (non-crystalline); Cleavage: None (conchoidal fracture); Specific Gravity: 2.35-2.6

Hardness: 5-5
Color: Typically black, can be grey, green, or mahogany
Luster: Vitreous (glassy)

Identified March 25, 2026More igneous →

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Physical properties

Formation & geological history

Formed from rapid cooling of felsic lava, preventing crystal growth. Usually found in margin areas of flows or at the edge of volcanic domes. Geological age varies but usually young (Cenozoic) because glass eventually devitrifies into crystals.

Uses & applications

Used for making jewelry (cabochons), scalpel blades (it can be sharper than steel), and ancient tools such as arrowheads and mirrors.

Geological facts

Obsidian is technically not a mineral because it lacks a crystalline structure; it is a 'mineraloid'. Ancient Aztec civilizations used obsidian for ceremonial 'macuahuitl' swords.

Field identification & locations

Identify by its characteristic conchoidal (shell-like) fracture and sharp edges. Common in volcanic regions like the American Northwest (Oregon/California), Mexico, and Iceland. Collectors look for 'Snowflake' or 'Rainbow' variants.

Other igneous specimens

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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