Rock Identifier
Black Obsidian (Volcanic Glass (SiO2 + impurities like Fe3O4)) — igneous
igneous

Black Obsidian

Volcanic Glass (SiO2 + impurities like Fe3O4)

Hardness: 5–5.5 Mohs scale; Color: Jet black; Luster: Vitreous/glassy; Structure: Amorphous (non-crystalline); Cleavage: Conchoidal (shell-like fracture); Specific Gravity: 2.35–2.6

Hardness
5–5
Color
Jet black
Luster
Vitreous/glassy
Identified More igneous →

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Physical properties

Hardness: 5–5.5 Mohs scale; Color: Jet black; Luster: Vitreous/glassy; Structure: Amorphous (non-crystalline); Cleavage: Conchoidal (shell-like fracture); Specific Gravity: 2.35–2.6

Formation & geological history

Formed from extrusive igneous activity when silica-rich lava cools extremely rapidly, preventing crystal growth. Most specimens observed today are less than 20 million years old, as older glass eventually devitrifies.

Uses & applications

Used in gemstone and jewelry making (cabochons), scalpel blades for high-precision surgery, decorative figurines, and metaphysical collecting.

Geological facts

In pre-Columbian times, Aztec and Mayan cultures used it for 'Macuahuitl' (swords) and mirrors. It is technically a 'mineraloid' rather than a mineral because it lacks a crystalline structure.

Field identification & locations

Identify by its characteristic conchoidal fractures which create razor-sharp edges and its uniform, glassy surface. Commonly found in volcanic regions like the American West (Glass Buttes), Mexico, and Iceland.

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