In one paragraphObsidian is a naturally occurring volcanic glass, not a mineral — it is rhyolitic lava that cooled so fast its SiO2-rich melt never had time to crystallise into quartz, feldspar, or mica. Its conchoidal fracture once made it the sharpest cutting tool on Earth. The black is iron oxide; the snowflake patches are cristobalite spherulites; the rainbow and gold sheen come from nanometre-thin layers and aligned hematite flakes that diffract light.
Most people meet obsidian first as a souvenir paperweight — a glossy black flake with edges sharp enough to cut paper — and assume it is a stone. It is closer to bottle glass than to quartz. The same eruption that makes a pumice ridge can, ten metres deeper, freeze a viscous lava into the most translucent natural glass on Earth. Run a thumbnail along a fresh edge and you will feel why Mesoamerican knappers preferred obsidian over flint for over four thousand years.
This guide explains what obsidian actually is, why its varieties (black, snowflake, rainbow, gold sheen, mahogany, Apache tears) look so different, where the visible material on the market forms, and what to look at when you assess a strand. It is written for buyers who want to read a piece before they own it.
What obsidian actually is
Obsidian is a mineraloid, not a mineral. A mineral has a defined chemical formula and an ordered atomic lattice; obsidian has neither. It is an amorphous solid — a frozen liquid. Chemically it is dominated by silica (typically 70-75% SiO2), with a few percent each of Al2O3, Na2O, K2O and trace Fe, Mg, Ca, Ti.
It forms when felsic (silica-rich) lava is extruded at the surface and quenched in days or weeks rather than the thousands of years a granite needs to crystallise. The melt is so viscous — silica polymers tangle like wet hair — that water and volatiles cannot diffuse out fast enough to nucleate quartz or feldspar crystals. The result is a homogeneous glass with no cleavage planes, fracturing instead in smooth curved shells (conchoidal fracture). A Mohs hardness of 5 to 5.5 puts it well below quartz (7); a strand of obsidian beads scratches more easily than a strand of clear quartz, which surprises many first-time buyers.
Obsidian is metastable. Over geological time (millions of years) the glass slowly devitrifies into microcrystalline quartz, feldspar and cristobalite — which is why obsidian older than about 20 million years is extremely rare in the field.
Why the colour and effect vary
Pure rhyolitic glass is colourless to pale grey. Everything else — the deep black, the moon-white feathers, the gold flash, the rainbow halo — comes from nanoscale inclusions and structures suspended in the glass. Each variety is a different geological accident.
| Variety | Inclusion / structure | What it tells you |
|---|---|---|
| Black obsidian | Disseminated magnetite and hematite (Fe oxide) nanocrystals (<1% by mass) | Standard high-silica glass with normal iron content; the most common variety worldwide |
| Snowflake obsidian | White cristobalite spherulites (radial SiO2 crystallites grown after solidification) | The glass is mid-devitrification; the snowflakes are the first crystals forming from the amorphous matrix |
| Rainbow obsidian | Aligned nanometre-thick layers of magnetite nanoparticles | Thin-film interference (light bouncing between layers) — the colour shifts with viewing angle, like a soap bubble |
| Gold sheen / silver sheen | Aligned hematite or magnetite microflakes oriented by lava flow | A single direction of flow during cooling; the sheen is a chatoyant reflection from parallel platelets |
| Mahogany obsidian | Iron oxide bands (rust to brown) in a black glass matrix | Two pulses of contaminated lava mingled before quenching; the banding is the flow record |
| Apache tears | Small transparent to translucent nodules of obsidian in a softer perlite host | Nodules of fresh glass that survived hydration; backlit they glow smoky brown |
Where the visible material forms
Obsidian needs a specific recipe: high-silica magma, surface or near-surface extrusion, and rapid cooling. That recipe is most often met at felsic stratovolcanoes and rhyolite domes, which is why the world's significant deposits cluster around active or recently active tectonic margins.
| Origin | Typical character | What to look for |
|---|---|---|
| Glass Buttes, Oregon, USA | Rainbow and sheen varieties from a 5-7 Ma rhyolite dome complex | The most reliable source of true rainbow obsidian; vivid green and purple flashes at low angles |
| Hidalgo, Mexico (Pachuca district) | Gold sheen and silver sheen from late-Miocene rhyolitic domes | Even sheen across the whole bead, not a single bright stripe — that is the sign of well-aligned platelets |
| Lipari, Aeolian Islands, Italy | Mahogany and banded obsidian from the Rocche Rosse flow (~1220 AD) | Sharp colour bands; geologically very young material, still chemically fresh |
| Armenia (Geghasar, Gutansar) | Black to grey-black obsidian from Pleistocene rhyolite domes | Archaeologically traceable by trace element fingerprint; tools made here travelled across the ancient Near East |
| Mendoza, Argentina (Cerro Huenul) | Black and snowflake obsidian from Miocene-Pliocene volcanic centres | Distinct cristobalite spherulites with sharp, snowflake-like edges — not blurred patches |
Reading an obsidian strand
Most retail obsidian is opaque black, which makes assessment harder than for a transparent quartz. The signal lives in surface and edge behaviour, not internal clarity.
- Lustre. A well-finished bead has a near-vitreous (glass) lustre, almost wet-looking. A dull, plasticky sheen usually means the polish stage was rushed or the bead has been heavily reconstituted.
- Conchoidal scars. Tiny shell-shaped chips on the drill hole are normal — obsidian is brittle. What you do not want is a long radial crack running from the hole into the body of the bead.
- Sheen evenness (gold / silver / rainbow). Rotate the strand under a single light source. The sheen should travel as a continuous band, not flash on three beads and vanish on the next.
- Snowflake distribution. Real cristobalite spherulites are three-dimensional — you can see them resolve into a radial pattern under a 10x loupe. Painted or dyed imitations sit flat on the surface.
- Backlight. Hold a piece against strong light. Most black obsidian transmits a deep mahogany glow at the edges; full opacity sometimes signals a glass-and-resin composite.
Trade names, decoded
Obsidian carries some of the most varied trade vocabulary in the strand market. Most names are descriptive of the optical effect, not a separate mineral species.
- Apache tears. Small (1-3 cm) translucent obsidian nodules from Arizona and New Mexico, weathered out of a perlite (hydrated glass) host. Same chemistry as bulk obsidian, just smaller and more transparent.
- Black obsidian. The default — high-silica glass with disseminated iron oxide. Look for true vitreous lustre rather than a flat, soot-like surface.
- Rainbow obsidian. Real rainbow obsidian shows iridescence under a single point light because of thin-film interference. Beads that show colour only under fluorescent strip lights are often dye-treated.
- Gold sheen / silver sheen obsidian. Same volcanic glass with a flow-aligned plane of microflakes. The two names just describe the colour of the reflection (Fe-rich vs Ti-rich flakes).
- Mahogany obsidian. Banded brown-and-black; iron oxide enrichment along flow lines.
- Sheen obsidian (generic). A catch-all retail term — ask whether it is gold, silver, or simply rainbow under a specific light.
Caring for obsidian
Obsidian's brittleness is the only real risk. Avoid ultrasonic cleaners — vibration can propagate microcracks along old conchoidal scars and split a bead. Clean with lukewarm water and a soft cloth; dry immediately. Store away from harder strands (quartz, agate, garnet) which will abrade the polish over time. Direct, prolonged sunlight is fine — obsidian does not fade — but sharp impacts on tile or stone floors will chip an edge instantly.
How BE. grades obsidian
Each strand is read against our Crystal 4T framework — Transparency, Tone, Texture and Trace — and shipped with the matching Stone Origin Card that names the source country and region (and the specific deposit where the upstream supplier has disclosed it), the variety, and the optical mechanism behind the colour. For gold sheen and rainbow varieties we check the sheen continuity bead-by-bead under a single point light: a strand passes only if the reflection travels as a continuous band across at least 80% of the beads. Mahogany strands are graded for band contrast and absence of devitrification cloudiness.
Frequently asked questions
Q1.Is obsidian a crystal?
No. Obsidian is a volcanic glass — an amorphous solid with no ordered atomic lattice. It is classified as a mineraloid alongside opal and amber. Quartz, by contrast, is a true crystal of SiO2.
Q2.How can I tell real obsidian from black glass?
Look at the edge of a chip under a loupe — real obsidian shows conchoidal fracture (smooth curved shells) and a very slight mahogany glow when backlit. Manufactured black glass is usually opaque, lighter in weight, and often has tiny rounded air bubbles.
Q3.Why is obsidian softer than quartz?
Obsidian sits at Mohs 5-5.5 because it lacks a crystal lattice — there is no preferred bonding direction to resist scratching. Quartz at Mohs 7 is harder despite being chemically similar, because its atoms are locked into a rigid framework.
Q4.Where does the rainbow in rainbow obsidian come from?
Thin-film interference. Aligned layers of magnetite nanoparticles, often only tens of nanometres thick, reflect light at different wavelengths from each layer. The visible colour shifts with viewing angle — the same physics as a soap bubble or oil slick.
Q5.Can obsidian get wet?
Briefly, yes. Long immersion is not recommended because over centuries obsidian slowly absorbs water (this is the basis of obsidian hydration dating), but daily wear and washing does no measurable damage. Just avoid ultrasonic cleaners.
Q6.Why does my snowflake obsidian look different on each bead?
Because cristobalite spherulites grow in three dimensions inside the glass. Each bead is cut from a different position in the host block, so the visible cross-section of the snowflake varies — a feature, not a defect.
References
- Mindat — Obsidian data
- GIA Gems & Gemology — Volcanic glasses
- Wikipedia — Obsidian
- Wikipedia — Cristobalite (snowflake mechanism)
- Webster, R. (2002). Gems: Their Sources, Descriptions and Identification, 5th ed. Butterworth-Heinemann.
- Schumann, W. (2009). Gemstones of the World, 4th ed. Sterling.
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