Spiderweb Obsidian Identification Guide
Identify spiderweb obsidian by its glassy black base laced with web-like gray devitrification veins, conchoidal fracture, and low density.
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What Spiderweb Obsidian Looks Like
Spiderweb obsidian is volcanic glass with a network of fine, web-like veins or fractures of lighter gray, white, or brownish material crossing a glossy black base. The web is formed by devitrification — partial crystallization of the glass along cracks and flow surfaces — producing thread-like patterns reminiscent of a cobweb. It is closely related to snowflake and mahogany obsidian, all being patterned volcanic glasses.
- Color: black base with gray/white/tan web veining
- Luster: vitreous (glassy), bright on polished faces
- Transparency: translucent on thin edges to opaque
- Habit: amorphous glass; massive
Step-by-Step Field-ID Checklist
- Confirm glassiness. A bright, glassy sheen and conchoidal fracture (curved, shell-like) are essential for obsidian.
- Examine the web. Veins are within the glass, often slightly recessed or different in texture (devitrified), tracing branching networks.
- Hold a thin edge to light — obsidian transmits light and glows at the margins.
- Heft test — it is light like glass, not heavy like a dense ore.
- Look for sharp, razor-like edges on any chips.
Key Diagnostic Tests
- Mohs hardness: ~5–5.5; it scratches a copper coin and is scratched by a steel file or quartz.
- Streak: white.
- Fracture: conchoidal — diagnostic of glass.
- No cleavage (amorphous).
- Density: low, ~2.35–2.6 g/cm³.
- Not magnetic; no acid reaction.
Common Look-Alikes and How to Tell Them Apart
- Snowflake obsidian: has discrete rounded white spherulite 'snowflakes' rather than connected web lines; spiderweb shows linked, vein-like patterns.
- Spiderweb jasper: opaque and much harder (6.5–7, scratches glass), with no glassy conchoidal fracture or translucent edges; obsidian is softer glass.
- Spiderweb turquoise: blue-green, soft (5–6), and not glassy.
- Black glass slag / man-made glass: may show round bubbles and uniform color; lacks natural devitrification webbing and flow structure.
- Banded/mahogany obsidian: shows brown flow bands rather than a connected gray web.
Where It Is Found
Spiderweb obsidian comes from rhyolitic obsidian flows, chiefly in the western United States — Oregon, Nevada, Utah, California, and Arizona — and other young volcanic regions such as Mexico. It forms where slow cooling and flow allowed devitrification to thread through the glass.
Frequently asked questions
How can you tell if it's real spiderweb obsidian?
It is natural volcanic glass: vitreous luster, conchoidal fracture with sharp edges, translucent thin edges, and low density, overlaid with a web of lighter gray devitrification veins running through (not on top of) the black glass.
What is the difference between spiderweb obsidian and snowflake obsidian?
Snowflake obsidian has separate rounded white spherulites, while spiderweb obsidian shows connected, branching vein-like patterns from devitrification along cracks.
Spiderweb obsidian vs spiderweb jasper — how do I tell them apart?
Obsidian is glass: softer (about 5.5), glassy, with conchoidal fracture and translucent edges. Spiderweb jasper is opaque quartz, harder (6.5–7), and scratches glass.
What causes the webbing in spiderweb obsidian?
Partial devitrification — tiny crystals nucleating along fractures and flow surfaces in the cooling glass — creates the lighter, web-like veining.
Is spiderweb obsidian man-made?
No, genuine spiderweb obsidian is natural volcanic glass. Man-made black glass usually shows round trapped bubbles and uniform color rather than natural devitrification webs and flow structure.