Obsidian is a natural glass formed when silica-rich lava cools too fast to crystallise. Because it is glass, every test that distinguishes crystalline gems from glass gives the same answer for obsidian and for a bottle: singly refractive, RI around 1.49, SG around 2.4, conchoidal fracture. The identification question is therefore not "is it glass?" but "is it natural glass?" — and that question is answered by what is inside it and how it looks.
What obsidian is
Volcanic glass, mostly silica with iron and other elements giving it colour. Properties: RI 1.48–1.51, SG 2.33–2.50, hardness 5–5.5, singly refractive with strain, usually black or very dark brown (translucent brown at thin edges), sometimes with sheen, banding or inclusions that define trade varieties.
The varieties and what makes them
- Black obsidian — the common form; opaque in mass, brown at thin edges.
- Snowflake obsidian — black with white to grey patches of cristobalite (a silica mineral) that crystallised in the glass. The patches are radiating, feathery, and irregularly distributed. Natural and distinctive.
- Mahogany obsidian — black with reddish-brown areas from iron oxide, in swirled flow patterns.
- Rainbow / sheen obsidian — a sheen or iridescent bands from oriented microscopic inclusions or gas bubbles, visible when the polished surface is angled to a light.
- Gold and silver sheen obsidian — metallic sheen from aligned gas bubbles.
- Apache tears — small rounded nodules of obsidian, translucent smoky brown.
Manufactured glass sold as obsidian
Black glass is cast into spheres, points, skulls and beads and sold as obsidian. Tells:
Bubbles. Manufactured glass typically has round bubbles, sometimes many. Obsidian can have bubbles too, but they are usually elongated along the flow and sparse.
Flow banding. Genuine obsidian shows fine, irregular flow banding and swirls of slightly different composition, visible in transmitted light at thin edges and on polished surfaces. Manufactured glass is uniform or shows the regular swirls of stirring.
Inclusions. Tiny crystallites (microlites) and, in the varieties, cristobalite patches, iron oxide areas and aligned inclusions that manufactured glass lacks. Fake snowflake obsidian with painted or moulded "snowflakes" shows patches that sit on the surface or are too regular.
Colour at thin edges. Natural black obsidian transmits brown; some manufactured black glass transmits green, blue or purple, betraying its colouring agents.
Surface. Cast pieces may show mould seams; polished obsidian does not.
Too perfect. Large, flawless, jet-black spheres of "obsidian" at low prices are usually glass; obsidian of that size and clarity is uncommon.
How the laboratory examines it
Confirms glass (RI, polariscope, SG) — which does not separate natural from manufactured. Then magnification for microlites, flow structure, bubble shape and inclusion character; transmitted-light examination of thin edges for colour and banding; comparison with reference obsidian. Report states: natural volcanic glass (obsidian), or manufactured glass, with the basis.
"Green obsidian", "blue obsidian" and other colours
Bright green, blue, red and other vividly coloured "obsidian" is manufactured glass, effectively without exception. Natural obsidian is black, brown, grey, or these with sheen and inclusions; genuinely green or blue natural obsidian is rare and never the bright bottle colours sold under the name.
Frequently asked questions
Is obsidian a mineral?
No — it is a glass, amorphous, without crystal structure, which is why it does not blink on the polariscope and why the usual crystal tests do not apply.
Is obsidian sharp?
Its conchoidal fracture produces edges sharper than steel, which is why it was used for tools. Handle broken pieces with care.
Does obsidian have a hardness test?
Around 5–5.5; it scratches with a steel knife. The laboratory does not scratch.
Is rainbow obsidian dyed or coated?
Genuine rainbow sheen is from inclusions inside the glass and moves with the angle of light. Coated or painted imitations show colour on the surface that does not shift.
Is snowflake obsidian manufactured?
Genuine snowflake obsidian is natural. Imitations with painted or moulded white patches exist; the patches lack the radiating cristobalite structure.