How to Recognise Real Obsidian: Natural Volcanic Glass or Imitation
Obsidian is natural volcanic glass. How to tell it apart from manufactured black glass and false blue obsidian, its varieties, and safe tests.
Key points
| Nature | natural volcanic glass (mostly silica, at least 70%), non-crystalline |
|---|---|
| Hardness (Mohs) | about 5 to 5.5 depending on sources (sometimes cited as 5 to 6) |
| Density | about 2.4 to 2.6 |
| Fracture | conchoidal: extremely sharp edges |
| Natural varieties | snowflake, golden and silver (sheen), rainbow, mahogany, sky eye |
| Main pitfall | manufactured black glass (industrial glass, slag); “blue obsidian” is most often artificial |
| Deposits | Mexico, Turkey, Italy (Lipari), Greece (Milos), Iceland, United States and others |
| Precaution | sharp edges; fragile to knocks and temperature changes |
A stone that is already glass
Obsidian is natural glass: silica-rich lava that cooled so quickly that no crystal had time to form. It is therefore amorphous, with a shell-like (conchoidal) fracture that produces extremely sharp edges. This is why prehistoric peoples made blades and points from it, and why the Aztecs made mirrors from it. It occurs near volcanoes of silica-rich lava: Mexico (Pachuca), Turkey, Italy (Lipari), Greece (Milos), Iceland, the United States and elsewhere.
This creates a difficulty specific to this stone: saying “it's glass” does not prove that it is fake. The real problem is manufactured black glass (glassworks offcuts, foundry slag, moulded glass) sold as obsidian. For the page on black stone, see black obsidian.
Natural varieties to know
- Black obsidian: the most common.
- Sky eye: grey to bluish-white bands or spots on black. See sky eye obsidian.
- Snowflake: white spots of cristobalite, a silica mineral, on a black background. See snowflake obsidian.
- Golden and silver: a metallic sheen produced by aligned gas bubbles. See golden and silver.
- Rainbow (also “fire obsidian”): an iridescent sheen due to tiny oriented inclusions; sources disagree on their exact nature (magnetite, hedenbergite or others).
- Mahogany: red-brown, with traces of iron oxide. See mahogany.
- Apache tear: small rounded, translucent nodules, often embedded in greyish perlite.
The natural blue or green shades of obsidian are subtle and only appear in transmitted light.
Warning signs
| Sign | Natural obsidian | Manufactured glass |
|---|---|---|
| Bubbles | Tiny, often stretched in the direction of lava flow | Round, regular bubbles, or too many |
| Texture | Flow bands, small paler areas, microcrystals | Perfectly uniform, bottle- or slag-like appearance |
| Thin edges | Smoky brown or grey, translucent under strong light | Strong green, blue or brown, depending on the glass |
| Vivid blue or green obsidian | Very unlikely: natural blues are barely perceptible | Coloured slag glass, sold as “blue obsidian” |
| Rainbow or eye | Iridescence from layers or inclusions, varying from place to place | Uniform surface iridescence (coating) |
| Shape | Rough, irregular; sharp conchoidal edges | Too regular, moulded edges |
Simple and safe tests
- Shine light through the thin parts (edges) with a strong lamp: natural obsidian turns grey or smoky brown and shows a cloud of micro-flaws; a vivid blue or green glass betrays artificial glass.
- Look at the bubbles under a magnifier: tiny and stretched for obsidian, round and clean for glass.
- Look for flow bands or crystal inclusions: a good sign of natural origin.
- Weigh and touch: obsidian feels cooler and is lighter than lead glass; plastic is clearly lighter.
- Compare with a stone of known natural colour or a known rough piece.
- Read the description: “natural obsidian”, “sky eye obsidian”, “black glass”.
Safety: handle rough chips with care. The edges are sharper than a steel blade; a damaged rough stone can cut. Avoid knocks, thermal shock and scratch or blade tests. Keep rough pieces out of the reach of young children.
Questions to ask before buying
Black obsidian is affordable: if the price is entirely comparable to that of decorative glass, nothing is unusual. However, obsidian in a vivid colour, a transparent sphere or an object with a very regular shape should prompt you to ask about its composition. Also ask about its origin (for example Mexico, Turkey, Italy): a precise provenance is a good sign of seriousness, even if it proves nothing by itself.
What these tests cannot prove
Some good-quality glasses (slag or moulded glass) imitate dark obsidian very well, and a very pure batch of natural glass may seem artificial. A laboratory measures density, refractive index and chemical composition to distinguish obsidian from manufactured glass. For a valuable object (an antique mirror, an archaeological piece, a signed piece of jewellery), it is best to seek a specialist's opinion.
In our shop, the nature of each piece is stated on its sheet, with indicative photos: see our commitments. For the “blue obsidian” variety or very vivid colours, assume the glass is artificial unless stated otherwise. The real or fake stone page gives the overview.
Frequently asked questions
Is obsidian glass?
Yes, it is natural volcanic glass. That does not make it fake: manufactured glass sold as obsidian is.
Does blue obsidian exist?
Natural blues are barely perceptible. A vivid, transparent blue obsidian is almost always artificial glass sold under that name.
How can you recognise rainbow obsidian?
The iridescent sheen varies from place to place and seems to come from internal layers. A perfectly uniform surface sheen suggests a coating.
Is obsidian dangerous?
Not because of its composition, but its rough chips are extremely sharp. Handle them with care and keep them out of children's reach. Polished stones do not present this risk.
What is an Apache tear?
A small rounded nodule of obsidian (marekanite), often translucent and embedded in perlite. The name has entered commercial use, particularly in the United States.
Also read
The meanings mentioned belong to tradition and symbolism. They are neither promises of results nor medical advice. Back to Recognizing real stones.