Banded Obsidian Identification Guide

What Banded Obsidian Looks Like

Banded obsidian is a natural volcanic glass that shows parallel or swirling stripes produced as viscous lava flowed and cooled too fast to crystallize. The bands range from black, gray, and brown to mahogany-red, and they often alternate with translucent zones. Key visual traits:

• Luster: bright, wet-looking vitreous (glassy) shine on fresh surfaces.
• Transparency: translucent on thin edges, opaque in the body.
• Form: massive, no crystals; bands are flow lines, not mineral layers.
• Fracture: smooth, curved conchoidal (shell-like) breaks with razor edges.

Field-ID Checklist

1. Confirm it is glass, not crystalline: look for total absence of grains, cleavage, or crystal faces.
2. Hold a thin edge to light to confirm translucency.
3. Look for flow banding that bends and swirls rather than straight depositional layers.
4. Strike a small flake (eye protection on) and check for conchoidal fracture with sharp edges.
5. Test hardness: glass scratches with a quartz point but not with a steel knife edge easily.

Key Diagnostic Tests

• Mohs hardness: about 5–5.5. A steel knife (5.5) barely scratches it; quartz (7) scratches it readily.
• Streak: white to colorless, regardless of body color.
• Fracture: conchoidal, never cleavage. This is decisive against most banded minerals.
• Density: ~2.35–2.6 g/cm³, light in the hand for a glassy stone.
• No reaction to dilute acid; not magnetic.
• Warmth: glass warms quickly in the hand compared with denser banded agate.

Common Look-Alikes and How to Tell Them Apart

• Banded agate: agate is crystalline chalcedony, hardness 7, and its bands are concentric depositional layers, often with a waxy rather than glassy luster. Agate scratches glass; banded obsidian does not scratch quartz.
• Manufactured glass / slag: can mimic the look but usually shows air bubbles, mold seams, or unnaturally vivid colors. Banded obsidian bands are flow-aligned and irregular.
• Apache tears: these are nodular obsidian (same material) but rounded and rarely banded; banding distinguishes the striped variety.
• Mahogany obsidian: essentially banded obsidian dominated by red-brown iron-stained streaks; the distinction is color emphasis, not material.
• Jet / black chert: jet is much lighter and warm, chert is dull and harder (7).

Where It Is Found

Banded obsidian forms at the margins and flow fronts of rhyolitic lava flows where shearing concentrates the banding. Notable sources include Glass Butte and the Cascade region of Oregon, the Mono-Inyo craters of California, the Jemez area of New Mexico, and volcanic zones in Mexico, Iceland, Armenia, and Japan. Look on eroded slopes and washes below young rhyolite domes, where weathering frees the glass.