Rainbow Obsidian Identification Guide

What Rainbow Obsidian Looks Like

Rainbow obsidian is a natural volcanic glass that appears solid black in ordinary light but reveals concentric rings or bands of green, gold, violet, and pink sheen when light strikes a properly oriented polished surface. The iridescence comes from microscopic aligned layers of magnetite nanoparticles within the glass, not from surface coating.

• Color: jet black base; sheen colors appear only at the right angle
• Luster: bright vitreous (glassy)
• Transparency: opaque to translucent on thin edges
• Habit: massive glass, no crystals; sold as polished cabs, spheres, and slabs

Step-by-Step Field ID Checklist

1. Rotate the polished piece slowly under a point light. True rainbow obsidian shows bands or rings of color that sweep across as you turn it; the effect depends entirely on orientation.
2. Check a thin edge — obsidian glows brown or translucent gray when backlit.
3. Inspect any chip. Look for smooth, curved, shell-like conchoidal fracture with razor edges.
4. Test hardness against a steel knife and glass.
5. Heft it — glass feels lighter than crystalline gem stones of equal size.

Key Diagnostic Tests

• Mohs hardness: 5–5.5. A steel knife (~5.5) barely scratches it; quartz scratches it easily.
• Fracture: conchoidal, with no cleavage — the single most reliable obsidian test.
• Streak: white to grayish.
• Specific gravity: ~2.35–2.6, low.
• Magnetism: generally not noticeably magnetic to a hand magnet despite the magnetite layers being microscopic.
• Acid: no reaction.

Common Look-Alikes and How to Tell Them Apart

• Gold-sheen / silver-sheen obsidian: these show a single uniform metallic sheen color; rainbow obsidian shows multiple banded spectral colors. All share glassy fracture.
• Black onyx / dyed chalcedony: harder (7), waxy luster, no rainbow sheen, no conchoidal glassy chip.
• Manufactured iridescent glass (slag/art glass): look for mold seams, perfectly uniform color, or trapped spherical bubbles; natural obsidian may contain irregular gas bubbles and flow lines but no mold marks.
• Labradorite/spectrolite: shows flash too, but is a feldspar with cleavage and hardness 6–6.5, not glassy conchoidal fracture.

Where It Is Found

Rainbow obsidian comes mainly from Mexico (Jalisco) and the western United States (Oregon, California, Arizona), forming where felsic lava cooled too quickly to crystallize.

Collector's Notes and Common Mistakes

Rainbow obsidian's sheen is orientation-locked, so a slab cut across the magnetite layering shows little or no color while a slab cut along it blazes — this is why two pieces from the same nodule can look wildly different. When buying rough, expect to grind and test orientations to find the best display angle. The biggest authenticity trap is manufactured iridescent "art glass"; check for mold seams, perfectly circular bubbles, and color that does not shift band-to-band the way natural magnetite sheen does. Do not confuse the surface rainbow with the discrete spectral flashes of opal or the metallic single-color flash of labradorite — obsidian's tell is always the glassy conchoidal fracture and low hardness (5–5.5). Because the edges are razor-sharp when freshly broken, handle chips carefully. Tumbled and cabbed pieces hold up well in jewelry but scratch more easily than quartz, so store them separately.