Natural volcanic glass. Not a mineral at all, strictly, and the hardest of all to separate from the manufactured kind.
Iceland, Mexico, Oregon, Armenia, Turkey — anywhere rhyolitic lava cooled fast6 field tests
Obsidian is lava that cooled too fast to crystallise, which makes it a glass rather than a mineral in the strict sense — it has no crystal structure and no fixed composition. That is also what makes it the hardest identification in this room, because the thing it is most often confused with is other glass, and glass is glass.
It breaks with the most perfect conchoidal fracture of any natural material, in curved surfaces that come to an edge sharper than surgical steel. This is why it was the most valuable trade good in the stone age across three continents, and it is also the property to respect when handling a freshly broken piece.
The honest position on identification is this: separating natural obsidian from manufactured black glass by eye is genuinely difficult, and a lot of 'rainbow obsidian' and 'electric blue obsidian' sold online is slag glass — an industrial by-product, which is a real material with its own following but is not obsidian. The colour is usually the clue, because natural obsidian's palette is narrow.
Because obsidian is amorphous, several of the usual tools are unavailable: no cleavage to read, no crystal habit, no consistent streak, no optical behaviour tied to a lattice. What is left is hardness, density, inclusions and colour range.
The inclusions are the most useful of these. Natural obsidian formed in a volcano, and volcanoes are dirty. Microscopic crystals of feldspar and magnetite, gas cavities stretched into elongated shapes by flow, and streaky flow banding are all normal. Manufactured glass is clean, or contains round bubbles.
| Property | Obsidian | What it separates it from |
|---|---|---|
| Mohs hardness | 5 – 5.5 | Quartz (7) — obsidian will not scratch a glass plate |
| Specific gravity | 2.3 – 2.6 | Hematite, jet and plastic all differ |
| Crystal structure | None — amorphous | Every true mineral |
| Fracture | Conchoidal, exceptionally clean, razor edges | Basalt and jet, which break dully |
| Lustre | Bright vitreous, almost wet-looking | Jet is more subdued; basalt is dull |
| Natural colour range | Black, grey, brown, mahogany, rare green or dark red | Bright blue, purple and 'rainbow': slag glass |
| Inclusions | Stretched gas cavities, microcrystals, flow banding | Round bubbles: manufactured glass |
| Translucency at a thin edge | Usually a warm brown or grey glow | Fully opaque black: consider jet or basalt |
Hold a thin edge up to a bright light. Most black obsidian transmits a brown or grey glow at the edge, which is a useful and quick sign — jet and basalt do not.
Ordered by how much each one proves. The decisive tests are worth your first two minutes; the supporting ones only matter once those agree.
Almost all obsidian is translucent at a thin edge, glowing brown, grey-green or occasionally deep red where the light gets through. Fully opaque black material with no edge transmission at all is more likely basalt, jet or black onyx. This is the fastest single check and it needs nothing but a lamp.
Under a loupe, natural obsidian's gas cavities are usually drawn out into lens or thread shapes by the flow of the lava, and often line up in a common direction with faint flow banding. Manufactured glass produces spherical bubbles, because a static melt has no reason to stretch them. Elongated, aligned cavities are a strong sign of natural origin.
Natural obsidian is black, grey, smoky brown, mahogany-and-black, and rarely a dark bottle green or deep red. It does not come in electric blue, violet, teal or a full spectrum. Vivid multicoloured 'rainbow obsidian' beads and spheres are usually slag glass — an industrial smelting by-product. Genuine rainbow obsidian exists, from Mexico, but its iridescence is a subtle sheen of bands seen at one angle in otherwise black stone, not bright body colour.
Obsidian is 5–5.5, and ordinary window glass is about 5.5. The two are close enough that obsidian will not cleanly scratch glass, and a steel knife will just mark obsidian. This matters mostly as a negative: black material that easily scratches glass is not obsidian, and is probably black tourmaline, onyx or another hard silicate.
Slag glass and moulded imitations often carry a faint seam, a slightly rounded facet edge, or a flat base where they sat while cooling. Natural obsidian is either rough or has been knapped or tumbled, and neither leaves a mould line.
A freshly broken obsidian edge is extraordinarily sharp — sharper than a steel scalpel, which is why it is still occasionally used in surgery. Run a fingertip alongside rather than across it. Manufactured glass is sharp too, so this is confirmation of the conchoidal fracture quality rather than a separation from glass.
The problem here is not deliberate counterfeiting so much as sloppy naming, and it has a specific centre: slag glass sold as obsidian. Slag is the vitrified waste from metal smelting, and it turns up in brilliant blues, greens, purples and mixed iridescent masses because it is full of metal oxides. It is a genuine collectible with its own history, and it is not volcanic.
The second and older issue is jet and black onyx being sold as obsidian, or the reverse. Jet is fossilised wood, much lighter in the hand at about 1.3 specific gravity, warm to the touch, and it will produce a brown streak on unglazed porcelain. Black onyx is dyed chalcedony, hardness 7, and will scratch glass where obsidian will not.
Each of these is checked differently. The ones with their own page have enough that is specific to them to be worth reading separately.
Red-brown and black in swirls, from iron oxidised within the glass.
Read within the checks aboveRounded translucent nodules weathered out of a soft ash matrix.
Read within the checks aboveSpecimen figures rather than gem figures, for material in honest condition. Most finds are worth very little, and saying so plainly is more useful than encouraging you.
| Form | What it typically fetches | Note |
|---|---|---|
| Rough obsidian, per kilo | $2 – 10 | Abundant wherever it occurs |
| Tumbled or polished piece | $3 – 15 | Snowflake and mahogany at the same level |
| Rainbow obsidian cabochon, well oriented | $20 – 80 | Skill of cutting, not rarity of material |
| Knapped modern blade or arrowhead | $15 – 150 | Priced as craft work |
| Genuine archaeological artefact | Legally fraught | Provenance and export law govern this entirely |
Obsidian is common and cheap as a material. What costs money is skilled cutting to show a sheen, and — quite separately — anything with archaeological standing, which carries legal restrictions rather than just a price.
Lovenhall StoneThe tests tell you what it is. The instrument reads a photograph and offers a name to start from — then hands you back to the tile and the coin, which are what settle it.