Obsidian Identification Guide

What Obsidian Looks Like

Obsidian is natural volcanic glass formed when felsic (rhyolitic) lava cools too fast to crystallize. It is amorphous (no crystal structure), so it has no mineral grains, no cleavage, and a brilliant glassy luster. Most obsidian is jet black, but it can be brown, gray, mahogany (red-brown streaked), green, or display sheen and rainbow effects from microscopic bubbles or inclusions. Edges are razor sharp. Snowflake obsidian shows white cristobalite spherulites.

• Color: black most common; brown, gray, mahogany, green, sheen/rainbow varieties
• Luster: bright vitreous (glassy)
• Transparency: translucent on thin edges (light passes through), opaque in mass
• Texture: smooth, homogeneous glass; no visible crystals

Step-by-Step Field ID Checklist

1. Look for glassy luster: a shiny, reflective, glass-like surface is the first clue.
2. Check for conchoidal fracture: smooth, curved, shell-like broken surfaces with very sharp edges — diagnostic of glass.
3. Backlight a thin edge: even "black" obsidian transmits light and looks translucent brown/gray at edges.
4. Confirm no crystals or vesicles: unlike basalt or pumice, obsidian is dense and grain-free (a few phenocrysts or bubbles can occur).
5. Test hardness: scratches glass (~5–5.5).
6. Heft: denser and heavier than frothy pumice.

Key Diagnostic Tests

• Hardness: ~5–5.5 on Mohs; will scratch a steel knife marginally and scratch window glass.
• Fracture: conchoidal, no cleavage — the hallmark of obsidian.
• Streak: white.
• Density: ~2.35–2.6 g/cm³; sinks in water (distinguishes it from floating pumice).
• Translucency: transmits light at thin edges.

Common Look-Alikes and How to Tell Them Apart

• Black glass slag / manmade glass: the hardest look-alike. Slag often has trapped round bubbles in regular sizes, unnaturally bright colors (blue, teal), and may be found near old smelters; natural obsidian has irregular flow features and occurs in volcanic terrain. Both are glass, so context matters.
• Apache tears: these ARE obsidian — small rounded nodules weathered out of perlite; translucent when backlit.
• Black tourmaline (schorl): crystalline with striated prismatic form and a defined shape; harder (~7) and not glassy-amorphous.
• Onyx/black agate: harder (~7), waxy chalcedony banding, not a true glass.
• Basalt: dull, fine-grained, often vesicular and not glassy; opaque even on edges.
• Jet: much lighter (low density), warm to touch, brown streak, can take a static charge — jet is fossil coal, not glass.
• Hematite/black tektites: tektites are also glass but rarer; hematite gives a red streak.

Where Obsidian Is Found

Obsidian forms at rhyolitic lava flows and volcanic domes of relatively young volcanic regions. Notable sources include the western United States (Glass Buttes and Newberry in Oregon, Obsidian Cliff in Yellowstone, Coso and Mono-Inyo in California), Mexico, Iceland, Italy (Lipari), Turkey, Armenia, and many Pacific Rim volcanoes. It is found as flows, nodules in perlite (Apache tears), and float; it weathers/hydrates over time, so very old obsidian may have a dull rind.