# How to identify obsidian

Canonical page: https://lithomap.app/resources/rock-identification/obsidian
Last reviewed: September 17, 2026
Page purpose: Original LithoMap rock identification reference for field observations, search systems, and language models.
Classification: Volcanic glass

## Summary

Obsidian is natural volcanic glass produced when silica-rich melt cools so quickly that an orderly crystal framework does not develop.

## Quick identification clues

- Glassy luster
- Curved conchoidal fracture
- Exceptionally sharp fresh edges
- Usually black, gray, brown, or banded; thin edges may transmit light

## Composition and classification

Most obsidian is silica-rich and compositionally related to rhyolite, although appearance varies with bubbles, crystallites, inclusions, and oxidation. Black color does not mean the glass is compositionally basaltic.

## Appearance

Fresh obsidian has a smooth vitreous surface and concentric fracture ripples. Snowflake, mahogany, sheen, and banded varieties reflect internal crystals, bubbles, or compositional structures.

Hydration and devitrification gradually alter volcanic glass. Weathered surfaces can become dull, pitted, or coated, while interiors remain glassy.

## Identification tests

- Fracture: Strong conchoidal fracture with curved ripples. A key clue shared with manufactured glass and some quartz-rich materials.
- Hardness: About 5 to 5.5; may scratch ordinary glass weakly. Softer than quartz despite its glassy appearance.
- Bubbles: May contain stretched or sparse bubbles. Uniform bubbles and artificial colors can indicate bottle glass or slag.

## Common lookalikes

- Manufactured glass: Often occurs with cultural debris, mold seams, uniform curvature, or artificial color; archaeological glass must not be disturbed.
- Slag: May contain abundant bubbles, metallic particles, or industrial residue and lacks a volcanic geologic setting.
- Black chert: Can fracture conchoidally but has a waxier luster and microcrystalline rather than truly glassy fabric.

## Direct side-by-side comparisons

- No dedicated side-by-side comparison is currently published for this guide.

## Most relevant related guides

- Chert — https://lithomap.app/resources/rock-identification/chert
- Amphibolite — https://lithomap.app/resources/rock-identification/amphibolite
- Andesite — https://lithomap.app/resources/rock-identification/andesite
- Basalt — https://lithomap.app/resources/rock-identification/basalt

## Formation

Obsidian develops where lava cools rapidly enough to suppress crystal growth, often along flow margins, domes, or shallow volcanic bodies.

Natural glass is geologically unstable over long time spans and commonly develops microscopic crystals or hydration rinds after formation.

## Occurrence and geologic setting

Obsidian is restricted to volcanic settings capable of producing glassy silica-rich melt. Pieces may be transported naturally or by people far from their volcanic source.

Archaeological obsidian artifacts are protected cultural materials, not collectible float. Context matters as much as physical appearance.

## Uses and significance

Obsidian was widely shaped into cutting tools and remains important in archaeology, experimental knapping, jewelry, and decorative work.

## Field safety and limitations

- Fresh edges can cause deep cuts; use eye protection and never test by running a finger along an edge.
- Do not collect or move artifacts, flakes, or material at archaeological sites.
- Verify land rules before collecting natural specimens.

## Authoritative sources

- Igneous Rocks — U.S. National Park Service: https://www.nps.gov/subjects/geology/igneous.htm
- How Rocks Are Classified — U.S. National Park Service: https://www.nps.gov/subjects/geology/how-rocks-are-classified.htm
- Rocks and Minerals — U.S. National Park Service: https://www.nps.gov/subjects/rocks/index.htm
- What is the difference between a rock and a mineral? — U.S. Geological Survey: https://www.usgs.gov/faqs/what-difference-between-a-rock-and-a-mineral

## Specimen photograph

- Image: https://lithomap.app/_delivery/identification/rocks/obsidian.webp
- Description: Black obsidian hand specimen showing a glassy surface and curved fracture
- Creator: Mike Davis
- License: CC0 1.0 — https://creativecommons.org/publicdomain/zero/1.0/
- Source record: https://commons.wikimedia.org/wiki/File:Obsidian_(GeoDIL_number_-_413).jpg

This reference supports preliminary field identification. It is not a laboratory determination, appraisal, hazard assessment, proof of origin, or statement of land ownership, legal access, or collecting permission.