How to identify obsidian and volcanic glass rocks
This guide explains clear, testable traits to tell obsidian and other volcanic glass from common rocks and man-made glass. Read the visual, tactile and simple field tests before using a scanner or photo tool for confirmation.
Quick facts
- Composition
- Obsidian: amorphous volcanic glass
- Fracture
- Conchoidal fracture surfaces
What obsidian is
Obsidian: amorphous volcanic glass. That means it cooled too quickly to form crystals or a mineral structure. It is volcanic glass rather than a crystal or traditional mineral, so its internal structure is not crystalline and it behaves like glass when it breaks.
Key visual traits
Obsidian is glassy and usually very dark, often black but sometimes brown, dark green, or banded. It has a smooth, shiny surface with glass-like luster. You will see Conchoidal fracture surfaces where it chips—curved, shell-like breaks with sharp edges. Polished pieces can look like gemstones or gem material but they are still volcanic glass.
Feel and simple field tests
Handle the piece carefully: fresh edges can be razor-sharp. It chips easily and produces curved flakes rather than flat cleavage planes. It is brittle rather than malleable. Use a magnifier or hand lens to check for lack of crystal faces; a true crystal will show planar faces or repeated structure, whereas obsidian will not. Do not rely on weight or shine alone to judge value or worth.
Common lookalikes and how to tell them apart
Man-made glass looks similar but often has bubbles or straight, uniform curvature from forming molds. Volcanic rocks with many vesicles (holes) are not obsidian. Pitchstone is also volcanic glass but has duller surface due to microscopic inclusions. If a sample shows visible mineral grains, crystal faces, or a layered sedimentary texture, it is not obsidian. A finder used to collect samples should note context: obsidian commonly occurs near volcanic vents, lava domes, and in association with other volcanic rocks.
Using photos, scanners and when to seek help
Take a clear photo showing any broken edge, the surface luster, and the context where you found the piece. A scanner or high-resolution photo helps with recognition but cannot replace a physical check for conchoidal breaks and sharp flakes. If you need confirmation of identity or of gem-grade material, get a second opinion from an experienced rocks or gem specialist and consider submitting clear photos and field notes. For casual identification and quick recognition from images, you can try Rock Identifier.
Common questions
- Is obsidian a gemstone or a mineral?
- Obsidian can be cut and polished like a gemstone, but it is not a mineral because it lacks a crystalline structure. It is volcanic glass formed by rapid cooling.
- How do Conchoidal fracture surfaces help identify it?
- Conchoidal fracture surfaces produce curved, shell-like breaks and very sharp edges. Looking for those curved flakes or bulbous fracture scars is one of the simplest and most reliable field clues.
- Can a photo or scanner reliably determine value or worth?
- Photos and scanners help with basic recognition and showing surface details, but assessing value or worth requires physical inspection for flaws, thickness, and how the piece will cut or polish. Use images for initial ID and expert input for valuation.