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.

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.

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.

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.

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).

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.

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.

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.