How to Identify Obsidian
Obsidian is natural volcanic glass — lava that cooled so fast it never grew crystals. Its calling cards are a smooth glassy texture, curved conchoidal fractures, and edges that can be sharper than a scalpel.
Quick Facts
- hardness
- 5–5.5 (scratches glass faintly; scratched by quartz)
- streak
- White
- luster
- Vitreous (glassy)
- color
- Usually black; also brown, mahogany, sheen, and rainbow
- crystal System
- None — amorphous glass
- cleavage
- None — conchoidal fracture
- specific Gravity
- 2.35–2.6
Where to Find It
Only in volcanic regions with a history of viscous, silica-rich (rhyolitic) lava. Look at and around lava domes and flows in places like the western US (Oregon, California, Yellowstone), Mexico, Iceland, and Italy. Obsidian weathers relatively quickly in geologic terms, so it is found near its volcanic source rather than far downstream.
What Obsidian Looks Like
- A smooth, uniform, glassy body with no visible crystals or grains — it looks like dark glass.
- Conchoidal fracture: curved, shell-like broken surfaces, often with concentric ripples.
- Extremely sharp edges on fresh breaks — obsidian flakes were used as blades and arrowheads for exactly this reason.
- Usually jet black, but also brown, mahogany (red-black streaks), 'snowflake' (gray cristobalite spots), and iridescent gold or rainbow sheen from microscopic inclusions.
- Translucent at thin edges — hold a flake to the light and its edge glows, often brownish, even when the piece looks solid black.
- A shiny, sometimes greasy-looking surface rather than the dull matte of most rock.
Simple Tests to Confirm It’s Obsidian
You can do these at home with basic tools. They won’t damage a good specimen.
Edge translucency test
Hold a thin edge or flake up to a bright light. Obsidian is translucent at the edges — light passes through and the rim glows, often brown. A completely opaque black material that blocks all light is more likely black chert, basalt, or slag.
Conchoidal fracture inspection
Examine broken surfaces. Obsidian breaks into smooth, curved, rippled hollows (conchoidal fracture) with knife-sharp edges and no flat crystal faces or grainy texture. This glassy, shell-like break is a defining feature.
Hardness / scratch test
Obsidian is about 5–5.5 — it will lightly scratch ordinary glass but is itself scratched by quartz (7) and a hard steel file. If your specimen is much harder (unscratched by quartz) or much softer (scratched by a fingernail or copper), it is not obsidian.
Uniform glassy texture check
Look with a hand lens for a homogeneous, crystal-free interior. Obsidian is glass, so it lacks the interlocking mineral grains of true igneous rocks like basalt. Visible crystals, gas bubbles throughout, or a grainy texture argue against obsidian (bubbly glass is often slag).
Not sure if your specimen is Obsidian?
Get an AI-powered first guess from a photo, then use the tests above to confirm.
Look-Alikes & How to Tell the Difference
Slag glass (man-made)
Industrial byproduct glass that mimics obsidian, but slag usually shows abundant air bubbles, flow lines, and unnatural colors (blue, green, teal). Obsidian rarely has through-body bubbles and comes only from volcanic areas — context and bubbles are the tell.
Black chert / flint
Also conchoidal-fracturing and can be black, but chert is opaque (no edge translucency), harder (7 vs obsidian's ~5.5), and duller/waxier rather than glassy. Shine a light on an edge: chert stays dark, obsidian glows.
Onyx (banded chalcedony)
Black onyx is microcrystalline quartz — harder (7), often faintly banded, and lacks obsidian's pure glassy fracture. It will scratch glass more decisively and is not tied to volcanic sources.
Tourmaline (schorl)
Black tourmaline can look glassy but forms elongated striated crystals and is harder (7–7.5). Its crystal habit — long prisms with lengthwise striations — is completely unlike obsidian's structureless glass.
Frequently Asked Questions
How do I know if a black rock is obsidian?+
Check three things: hold a thin edge to the light — obsidian is translucent and glows at the edge; look for smooth, curved conchoidal fracture with very sharp edges; and confirm a glassy, crystal-free texture. Add a volcanic-region context and no through-body bubbles, and you have obsidian rather than chert, basalt, or slag.
How is obsidian different from regular volcanic rock like basalt?+
Basalt cooled slowly enough to grow tiny interlocking crystals, so it is an opaque, dull, granular rock. Obsidian cooled so fast it stayed a glass — no crystals, glassy luster, conchoidal fracture, and edge translucency. In short, basalt is crystalline igneous rock; obsidian is natural glass.
How can I tell obsidian from man-made glass or slag?+
Slag and manufactured glass tend to be full of air bubbles, show flow lines, and come in unnatural colors like bright blue or green. Obsidian rarely has bubbles running through it, is usually black to brown, and is found near volcanoes. If a 'black glass' is bubbly and turned up in an old industrial area, suspect slag.
Is obsidian dangerous to handle?+
Fresh obsidian edges are genuinely razor-sharp — sharper than surgical steel — and can cut skin easily. Handle flakes and freshly broken pieces carefully, avoid running a finger along an edge, and keep them away from children. This sharpness is why obsidian was prized for blades and arrowheads.
Can a photo app identify obsidian?+
A photo app can recognize obsidian's glassy black look and conchoidal fracture and give a good first guess, but a camera cannot judge edge translucency or hardness, and slag glass looks similar. Confirm with the flashlight edge test and a quick hardness check — those separate real obsidian from its impostors.
Related identification guides
This field guide is educational. Visual identification and simple at-home tests are helpful first steps, but they are not a certified geological, gemological, or meteorite analysis. Verify valuable, hazardous, or scientifically important finds with a qualified expert.