Iridescent Obsidian Identification Guide

What Iridescent Obsidian Looks Like

Iridescent obsidian is volcanic glass that displays a metallic sheen or rainbow play of color when light hits aligned microscopic inclusions (often nanoscale magnetite crystals or gas bubbles arranged in flow layers). At rest it looks like ordinary black obsidian; tilted to the light, bands of gold, green, violet, or full spectrum appear.

• Color: base is black to dark brown; sheen colors include gold, silver, green, blue, violet, or rainbow
• Luster: vitreous (glassy), high
• Transparency: translucent on thin edges, otherwise opaque
• Form: massive, no crystals — it is amorphous glass with conchoidal (shell-like) fracture

Step-by-Step Field-ID Checklist

1. Confirm it's glass. Look for conchoidal fracture — smooth, curved, shell-like breaks with razor edges. This is the hallmark of obsidian.
2. Tilt under a single light source. The sheen or rainbow should sweep across the surface as you rotate it, then vanish at other angles. The color lives inside the stone, not on the surface.
3. Check the base color by looking at a thin edge against bright light — usually translucent grey-brown.
4. Feel the weight and warmth. Obsidian is glass: it feels relatively light and warms in the hand faster than crystalline stone.
5. Test hardness — about 5–5.5; it will scratch with a steel file and can scratch glass marginally.

Key Diagnostic Tests

• Mohs hardness: 5–5.5
• Streak: white
• Fracture: conchoidal — diagnostic for volcanic glass
• Cleavage: none (amorphous)
• Specific gravity: ~2.35–2.6 (low, glassy feel)
• Magnetism: generally none macroscopically, though magnetite nanoparticles cause the sheen
• No acid reaction

Common Look-Alikes and How to Tell Them Apart

• Labradorite / spectrolite: also flashes color, but labradorite is a crystalline feldspar with cleavage planes (flat reflective faces) and is harder (6–6.5). Obsidian has no cleavage and fractures conchoidally.
• Manmade slag glass / 'goldstone': goldstone shows tiny uniform copper sparkles throughout, not a directional sheen, and often has a too-perfect, evenly speckled look. Slag glass may show mold seams or bubbles in regular patterns.
• Black tourmaline or hematite: these are crystalline with no internal rainbow sheen and (for hematite) a red-brown streak.
• Rainbow moonstone: translucent white body with blue adularescence, not a black glassy base.

Where Iridescent Obsidian Is Found

Obsidian forms where felsic (rhyolitic) lava cools too fast to crystallize. Iridescent and rainbow varieties come notably from Mexico (Jalisco), the western USA (Oregon, Arizona, Glass Buttes), and Armenia. Sheen-grade material is selected and oriented by cutters to maximize the optical effect.