Obsidian — Volcanic Glass with a Memory of Fire
Obsidian is lava that cooled so quickly it never had time to crystallize. The result is natural glass—smooth, shiny, and capable of breaking into edges sharp enough to make a geologist quietly say “careful.” In hand it’s midnight with a mirror’s polish; on the microscopic level it’s an elegant tangle of frozen melt. If rock had a minimalist phase, this would be it.
Identity & Naming 🔎
What it is
Obsidian is natural volcanic glass, an amorphous (non‑crystalline) solid formed by rapid cooling of silica‑rich lava. Because it lacks a long‑range crystal structure, it’s classified as a mineraloid rather than a mineral.
Hydration & aging
Fresh obsidian contains small amounts of dissolved water. Over time, a surface hydration rind develops as water diffuses in; under certain conditions the glass partially devitrifies into perlite—a light, popcorn‑prone rock used in potting soils.
How It Forms & Textures 🌋
Quenching felsic lava
Obsidian forms on the margins of rhyolite domes and flows, along lava coulees, and around shallow intrusions that cool quickly. Rapid heat loss prevents atoms from arranging into crystals—glass results.
Flow banding & microlites
As lava moves, tiny crystals (microlites) and melt layers align, producing flow bands—subtle ribbons that catch the light. Under a loupe they appear as wispy, parallel streaks.
Devitrification & snowflakes
With time or gentle heating, silica can crystallize within the glass as radial spherulites of cristobalite—creating snowflake obsidian. Onion‑skin perlitic cracks may form as hydration and cooling stress build.
Sheen & rainbow effects
Sheen obsidian (golden/silvery) glows from thin layers of tiny bubbles aligned in the glass. Rainbow obsidian shimmers with interference colors from nanoscale layers of inclusions—geology’s subtle hologram.
“Apache tears”
Small, rounded obsidian nodules weathered from perlitic tuffs. Held to light, they turn translucent brown like strong tea—always a delightful reveal.
Perlite: the afterlife
Hydrated obsidian can expand into frothy white perlite when heated—literally popping into lightweight kernels used in horticulture and lightweight concrete.
Short story: molten silica, a sudden chill, and a lifetime frozen mid‑flow.
Colors & Varieties 🎨
Palette
- Black — classic, often with subtle brown in strong light.
- Mahogany — warm brown/black swirls (iron oxides).
- Smoke/steel/green — trace element flavor and bubble density.
- Snowflake — gray‑white spherulites in black glass.
- Golden sheen — internal bubble sheets reflect warm light.
- Rainbow — interference colors in concentric arcs.
Surface & fracture
- Vitreous luster like polished glass.
- Conchoidal fracture makes curved “shell” breaks and ultra‑keen edges.
- Transparency: opaque to translucent on thin edges (tea‑brown).
Photo tip: Side‑light at ~30° reveals flow bands; a white bounce card opposite the light softens glare and deepens blacks.
Physical & Optical Properties 🧪
| Property | Typical Range / Note |
|---|---|
| Composition | Silica‑rich melt (rhyolitic); SiO₂ ~70–78% plus Al, Na, K, Fe, trace elements |
| Structure | Amorphous (no long‑range order) → mineraloid |
| Hardness | ~5–5.5 (can scratch common glass; chips easily) |
| Specific gravity | ~2.30–2.45 |
| Cleavage / Fracture | No cleavage; conchoidal fracture |
| Refractive index | ~1.48–1.51 (varies with composition) |
| Luster | Vitreous; resinous on weathered surfaces |
| Streak | White (powder); rarely used—streak plate is harder and will mark the glass |
| Magnetism | Non‑magnetic (unless iron‑rich inclusions present) |
Under the Loupe / Microscope 🔬
Flow textures
Look for parallel wisps and streaky bands—microlites and tiny bubbles aligned by flow. These give a satin sheen under raking light.
Spherulites & perlite
Snowflake spherulites show delicate, radiating needles. Perlitic cracks appear as concentric, onion‑skin fractures tracing hydration fronts.
Sheen & rainbow
Under magnification, sheen comes from bubble sheets; rainbow from ultra‑thin layered inclusions causing interference colors—both shift with viewing angle.
Look‑Alikes & How to Tell 🕵️
Black flint/chert
Also conchoidal, but harder (~7) and often slightly waxy rather than glassy. Flint commonly shows lighter cortex and sedimentary context.
Basalt
Fine‑grained igneous rock with tiny crystals; duller luster; no glassy translucency at edges. Basalt often contains visible feldspar or pyroxene microlites.
Black onyx & jade
Onyx is banded chalcedony (microcrystalline quartz) and much harder; jade (nephrite/jadeite) is tougher with fibrous/granular texture and no conchoidal shells.
Tektites
Impact glass: matte, pitted surfaces (“lechatelierite” textures), aerodynamic shapes. Obsidian is usually glossier with flow banding from lava movement.
Industrial slag
Can mimic black glass but often has bubbly, ropey textures and metallic sheen streaks. Context (near old furnaces/rail lines) is a clue.
Quick checklist
- Vitreous, mirror‑like luster.
- Conchoidal shells with razor edges.
- Translucent tea‑brown on thin edges (most specimens).
Localities & Archaeology 📍
Where it’s found
Obsidian rims many felsic volcanic centers worldwide: Mexico (Pachuca, Ucareo), USA (Yellowstone, Glass Buttes OR, Newberry, California), Iceland, Turkey (Cappadocia), Italy (Lipari, Pantelleria), Japan, Armenia, Ethiopia, and beyond.
Trade & tools
Prehistoric cultures knapped obsidian into blades, points, and mirrors. Chemical “fingerprinting” (trace‑element geochemistry) lets archaeologists trace artifacts to their volcanic sources, mapping trade routes across ancient landscapes.
Care & Safety 🧼
Handling
- Edges are blade‑sharp. Handle polished chips and fresh breaks with respect (and fingers out of the fracture line).
- Obsidian is brittle; avoid hard knocks and drops.
Cleaning
- Lukewarm water + mild soap + soft cloth; rinse and dry.
- Avoid rapid temperature swings—glass doesn’t love thermal shock.
Storage & display
- Store separately from harder quartz/corundum to keep the polish crisp.
- Side‑light around 30° highlights flow bands and sheen effects.
Hands‑On Demos 🧪
Conchoidal shell
Examine a broken edge under strong side‑light and trace the ripples from the impact point. Each ripple is a frozen shock wave in glass.
Tea‑brown translucency
Hold a thin edge in front of a flashlight: many “black” pieces glow brown to smoky gray. It’s a quick, satisfying check that you’re handling volcanic glass.
Small joke: obsidian doesn’t hold grudges—it just holds an edge.
Questions ❓
Is obsidian a mineral?
No. It’s a mineraloid (natural glass) because it lacks a repeating crystal lattice.
Why does obsidian sometimes look iridescent?
Thin, evenly spaced layers of bubbles or nanoparticles inside the glass cause light to interfere, creating sheen or rainbow colors that shift with angle.
Can obsidian be truly transparent?
Rarely in thick pieces. Thin chips and Apache tears can be translucent to nearly transparent brown.
Does it scratch easily?
It’s moderately hard (~5–5.5) but not tough. It resists gentle abrasion but chips with sharp blows—think window glass, not granite.
What’s the difference between obsidian and perlite?
Perlite is hydrated obsidian that became full of tiny water‑rich shells. Heated quickly, it puffs into white granules—gardens love it.