Porphyritic Obsidian Identification Guide
How to identify porphyritic obsidian by its glassy black groundmass studded with crystals, conchoidal fracture, and how to separate it from snowflake obsidian and basalt.
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What Porphyritic Obsidian Looks Like
Porphyritic obsidian is volcanic glass (obsidian) containing visible crystals (phenocrysts) set in a glassy groundmass. The base is typically glossy black (sometimes brown, gray, or mahogany) with a bright vitreous, glassy luster, and it hosts scattered feldspar, quartz, or pyroxene crystals plus tiny needle-like microlites. When the crystals are spherulites of cristobalite/feldspar, the rock grades into snowflake obsidian.
Step-by-Step Field ID Checklist
- Confirm a glassy base. Look for the brilliant, glassy luster and translucency on thin edges — obsidian glows brownish at the rim.
- Spot the phenocrysts. Identify discrete crystals or white/gray feathery spots scattered in the glass.
- Break a fresh edge. Obsidian shows classic smooth, curved conchoidal fracture with razor-sharp edges.
- Test hardness. Scratches glass (it is harder than window glass, ~5–5.5).
- Heft and feel. Lighter and warmer to the touch than dense crystalline rock; smooth, glassy surface.
Key Diagnostic Tests
- Mohs hardness: About 5 to 5.5.
- Cleavage: None; pronounced conchoidal fracture (the diagnostic obsidian trait).
- Streak: White.
- Specific gravity: About 2.35 to 2.6 (relatively light).
- Magnetism: Generally non-magnetic to weak.
- Luster: Bright, glassy vitreous — key separator from dull volcanic rock.
Common Look-Alikes and How to Tell Them Apart
- Snowflake obsidian: A type of porphyritic/spherulitic obsidian with white cristobalite snowflakes; the distinction is gradational, but snowflake obsidian's spots are radiating spherulites.
- Basalt: Dull, opaque, fine-grained crystalline (not glassy), heavier, and may be magnetic; obsidian is glassy with conchoidal fracture.
- Black glass slag (man-made): Often has bubbles and a different color sheen; lacks natural phenocrysts and may show mold textures.
- Apache tears: Rounded glassy nodules of obsidian; porphyritic obsidian is massive with embedded crystals.
- Black agate/onyx: Harder (7), waxy not glassy, and shows banding.
Where Porphyritic Obsidian Is Found
Porphyritic obsidian forms at the margins of rhyolitic lava flows and domes where rapid cooling traps earlier-formed crystals in glass. Notable localities include the western United States (Oregon, California, Idaho, Arizona), Mexico, Iceland, and Italy (Lipari).
Frequently asked questions
How can you tell if it's real porphyritic obsidian?
Look for a glassy black groundmass with a bright vitreous luster, visible embedded crystals (phenocrysts), conchoidal fracture with sharp edges, hardness about 5 to 5.5, and translucency at thin edges.
What does porphyritic obsidian look like?
It is glossy black (or brown/gray) volcanic glass studded with scattered feldspar, quartz, or pyroxene crystals and tiny needle-like microlites.
Porphyritic obsidian vs basalt: how do you tell them apart?
Obsidian is glassy with a vitreous luster and conchoidal fracture and is fairly light, while basalt is dull, opaque, crystalline, heavier, and sometimes magnetic.
Is porphyritic obsidian the same as snowflake obsidian?
They overlap. Snowflake obsidian is a spherulitic form with white crystal snowflakes, while porphyritic obsidian more broadly contains visible phenocryst crystals in glass.
Porphyritic Obsidian identified by the community
Recent Porphyritic Obsidian specimens identified with Rock Identifier.
