Brown Obsidian Identification Guide
Field identification of brown obsidian, a natural volcanic glass colored by iron, using fracture, hardness, luster, and look-alike comparisons.
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What Brown Obsidian Looks Like
Brown obsidian is natural volcanic glass tinted brown to root-beer, golden-brown, or smoky tan by dispersed iron (and tiny hematite/magnetite microlites). The body has a bright vitreous (glassy) luster and is translucent to opaque—thin edges often glow warm brown when backlit. Some pieces grade into mahogany obsidian (with red-brown swirls) or show patchy cloudy zones.
Crystal habit / form
Obsidian is amorphous—no crystals, no cleavage. The single most diagnostic feature is its conchoidal fracture: smooth, curved, shell-like break surfaces that produce extremely sharp edges. It occurs as massive flows, nodules, and rounded "Apache tear" pebbles.
Step-by-Step Field ID Checklist
- Backlight it. A warm brown glow through thin edges signals glass colored by iron.
- Check fracture. Look for smooth, curved conchoidal surfaces and razor edges.
- Confirm glassy luster—wet, glossy shine, not grainy.
- Test hardness. Mohs ~5–5.5; quartz scratches it, it barely scratches glass.
- Heft it. Light for size (~2.4 g/cm³) compared with metallic minerals.
- Look for flow lines or bubbles trapped in the glass.
Key Diagnostic Tests
- Mohs hardness: ~5–5.5. Scratched by quartz and hardened steel; softer than jasper/agate.
- Streak: White to pale gray.
- Fracture: Conchoidal, no cleavage—the defining glass test.
- Density: ~2.35–2.5 g/cm³.
- Magnetism: Usually none; magnetite-rich pieces may show a faint response.
- Acid: Inert.
Common Look-Alikes and How to Tell Them Apart
- Smoky quartz: Brown and translucent but crystalline, harder (Mohs 7), often with crystal faces; it scratches glass easily and obsidian does not.
- Brown jasper/chert: Opaque, harder (6.5–7), waxy rather than glassy, lacking the glassy conchoidal sheen of obsidian.
- Mahogany obsidian: Same material with red-brown streaks; brown obsidian is more uniformly brown.
- Brown bottle glass/slag: Man-made glass also fractures conchoidally—look for mold marks, perfectly even color, or seam lines indicating manufacture.
- Amber: Much softer (Mohs 2–2.5), warm, lightweight, and can float in saltwater—obsidian sinks and is far harder.
Where It Is Found
Brown obsidian forms from rapidly cooled high-silica (rhyolitic) lava. Sources include the western United States (Oregon, California, Idaho, Arizona, Utah), Mexico, Iceland, Armenia, and Japan. It is collected from lava flows and as water-rounded nodules; much hobby material is tumbled or knapped.
Frequently asked questions
How can you tell if it's real brown obsidian?
Real brown obsidian is natural glass: it shows conchoidal fracture with sharp edges, a glassy luster, hardness about 5–5.5 (quartz scratches it), a white streak, and a warm brown glow when held to light. Mold seams or perfectly uniform color suggest manufactured glass instead.
What is the difference between brown obsidian and smoky quartz?
Smoky quartz is crystalline, harder (Mohs 7), and often has visible crystal faces, while brown obsidian is amorphous glass at Mohs 5–5.5. Try scratching glass: quartz cuts it easily, obsidian barely does.
Brown obsidian vs mahogany obsidian—what's the difference?
They are the same volcanic glass; mahogany obsidian has distinct red-brown swirls or banding mixed with black, while brown obsidian is more evenly brown to root-beer colored.
What does brown obsidian look like?
It looks like glossy brown to golden or root-beer colored glass, translucent at the edges, with smooth curved fracture surfaces and sometimes trapped bubbles or flow lines.