Obsidian

Physical Properties

Hardness: 5-5.5 (Mohs scale); Color: Black to dark brown (translucent when thin); Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Cleavage: None (conchoidal fracture); Specific Gravity: 2.35-2.6

Formation & Geological History

Formed from rapid cooling of felsic/rhyolitic lava, which prevents crystal growth. This specimen appears andesitic or rhyolitic in composition and was likely shaped via knapping.

Uses & Applications

Used historically for sharp tools and weapons (arrowheads, knives); currently used in surgical scalpel blades, jewelry, and ornamental decor.

Geological Facts

Because it has no crystal structure, obsidian can be fractured down to an edge nearly a single molecule thick, making it sharper than high-quality steel scalpels used in medicine.

Field Identification & Locations

Identify in the field by its distinctive glassy texture and conchoidal (shell-like) fracture patterns. Found in areas of recent volcanic activity such as Iceland, Mexico, and the Western USA.

Identified on: 5/10/2026

Mode: Standard