Devitrified Obsidian Identification Guide

What Devitrified Obsidian Looks Like

Devitrified obsidian is volcanic glass in which part of the glass has crystallized (devitrified) into tiny radiating crystal clusters called spherulites. The result is a dark, glassy stone speckled with gray, white, or tan rounded patches — the most familiar form is snowflake obsidian, where cristobalite spherulites bloom like snowflakes in black glass.

Key visual cues:

• Body: black, gray, brown, or mahogany glassy matrix.
• Inclusions: rounded white/gray spherulites, sometimes feathery or star-like.
• Luster: vitreous (glassy) in the glass, duller in the crystallized patches.
• Transparency: translucent on thin edges of the glass; opaque spherulites.

Step-by-Step Field-ID Checklist

1. Confirm a glassy matrix. Look for conchoidal (shell-like) fracture and vitreous luster typical of obsidian.
2. Spot the spherulites — radiating, rounded crystal clusters, often whitish.
3. Test hardness. It scratches glass (Mohs ~5.5–6).
4. Check the break for sharp, curved conchoidal surfaces.
5. Hold a thin chip to light — the glass transmits light, the spherulites stay opaque.

Diagnostic Tests

• Mohs hardness: ~5–6 (glass-like; just scratches window glass).
• Streak: white.
• Fracture: conchoidal, producing razor edges.
• Density: ~2.35–2.6 g/cm³.
• Acid: inert to dilute HCl.
• Magnetism: generally none (rare magnetite specks aside).

Common Look-Alikes and How to Tell Them Apart

• Fresh (non-devitrified) obsidian: pure glassy, no spherulites; devitrified obsidian shows the crystallized blooms.
• Snowflake obsidian: is itself a devitrified obsidian — the named, snowflake-spherulite variety.
• Porphyritic obsidian: contains discrete feldspar phenocrysts (blocky crystals) rather than radiating spherulites.
• Perlite/pitchstone: perlite is pearly-gray and crumbly with concentric cracks; pitchstone has a resinous (not glassy) luster; devitrified obsidian keeps a glassy vitreous sheen.
• Flint/chert: waxier, no spherulites, and harder (Mohs ~7); obsidian is glassier and slightly softer.
• Manufactured glass/slag: lacks natural spherulites and often shows bubbles or mold marks.

Where It Is Found

Devitrified obsidian occurs in silicic volcanic flows wherever obsidian has had time and heat to begin crystallizing — older or slowly cooled rhyolitic glass. Notable sources include the western United States (Utah, Oregon, Arizona, California), Mexico, and other young volcanic regions worldwide. Snowflake obsidian from Utah is a classic example; collect it as float and flow blocks in volcanic terrain.