How to Identify Obsidian: What to Look For and Home Tests

Lava that cooled at the surface had no time to grow large crystals, so the rock is fine-grained or glassy, often with gas bubbles or a frothy top surface. The properties that identify Obsidian — luster: Glassy (vitreous) luster; color: Usually black; different oxidation states of iron can tint it red, brown, or green, often with visible banding or streaks; cleavage or fracture: No cleavage; conchoidal fracture, with edges that can be sharper than a fine steel blade. For a rock, texture and grain size matter more than any single hardness number.

• Fracture test: obsidian has no crystalline structure, so it fractures into smooth, conchoidal (shell-like) curved surfaces. Safety: Broken obsidian edges are extremely sharp. Always wear eye protection and heavy gloves when handling, striking, or knapping it to avoid cuts.

• [Granite](/lookup/rocks-and-minerals/granite): Obsidian is a glassy rock formed when rhyolite (the fine-grained, extrusive equivalent of granite) cools too quickly to crystallize.

Photograph a fresh, unweathered surface in daylight, then a second shot of any crystal faces or broken edge. Include a coin for scale. If you did a streak test, photograph the streak next to the specimen. The app suggests the likely mineral or rock; confirm with hardness and the tests above before you label it.

Obsidian forms from the rapid cooling of viscous, high-silica, low-water-content magma. Well-known North American sources include volcanic regions of Oregon and California (no citable .gov/.edu page giving specific distribution was found, so this is not included as a sourced field to avoid an unsupported claim).