Black Obsidian Identification Guide
Identify black obsidian, natural volcanic glass, by its glassy luster, conchoidal fracture, sharp edges, and how to separate it from black glass, jasper, and onyx.
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What Black Obsidian Looks Like
Black obsidian is a natural volcanic glass formed when felsic lava cools too fast to crystallize. It is amorphous (no crystal structure), with a brilliant glassy luster and a deep black body, often translucent brown-gray on thin chips.
- Color: jet black, sometimes smoky gray-brown on edges
- Luster: bright vitreous (glassy)
- Transparency: translucent on thin edges, opaque in mass
- Fracture: strongly conchoidal (curved, shell-like) with razor-sharp edges
- Features: may show flow banding, gas bubbles, or tiny crystallites (snowflakes, sheen)
Step-by-Step Field-ID Checklist
- Look for a conchoidal fracture. Smooth, curved, shell-shaped break surfaces with very sharp edges are the hallmark of glass.
- Check the luster. A bright, wet-looking glassy shine.
- Backlight a thin edge. Often glows translucent brown or gray.
- Hardness test. Obsidian is ~5–5.5; it scratches with difficulty and a steel file will mark it more easily than quartz.
- Heft it. Lighter than most rocks of similar size (SG ~2.35–2.6).
- Look for bubbles/flow lines. Natural obsidian may show them; perfectly clean material warrants caution.
Key Diagnostic Tests
- Mohs hardness: ~5–5.5 (softer than quartz; a quartz point scratches it).
- Streak: white.
- Cleavage/fracture: no cleavage; pronounced conchoidal fracture.
- Specific gravity: ~2.35–2.6 — noticeably light.
- No acid reaction; not magnetic.
- Texture: amorphous, no visible crystals or grains.
Common Look-Alikes and How to Tell Them Apart
- Manmade black glass / 'slag': the trickiest look-alike, since both are glass. Natural obsidian often shows flow banding, irregular bubble trails, and conchoidal chips; manufactured glass tends to have perfectly round, evenly spaced bubbles and uniform color. Mold seams indicate manufacture.
- Black jasper: crystalline quartz, harder (7), denser, opaque, duller natural surface.
- Black agate/onyx: chalcedony, harder (7), translucent with possible banding.
- Apache tears: these ARE obsidian — rounded translucent nodules weathered out of perlite.
- Basalt: dull, grainy, vesicular volcanic rock; not glassy.
- Jet: organic, much softer (2.5–4), very light, warm to the touch.
The key field discriminator is glassy luster + conchoidal fracture + hardness ~5 + light weight: quartz look-alikes are harder and denser, while basalt is dull and grainy. Separating obsidian from manmade glass relies on bubble/flow texture and the absence of mold seams.
Where Black Obsidian Is Found
Obsidian occurs at the margins of rhyolitic lava flows and domes in volcanic regions: the western USA (Oregon, California, Idaho, Arizona — including Apache tears), Mexico, Iceland, Italy (Lipari), Japan, Turkey, and Armenia. It weathers out as glossy black cobbles and is abundant at classic 'glass mountain' flows.
Quick Confirmation
A light, glassy black stone with a curved conchoidal fracture, sharp edges, hardness ~5, and brown translucency on thin edges is natural obsidian — provided it lacks the mold seams and perfectly uniform bubbles of manufactured glass.
Frequently asked questions
How can you tell if black obsidian is real?
Real obsidian is natural volcanic glass: it has a bright glassy luster, a curved conchoidal fracture with sharp edges, a hardness around 5–5.5, a light feel (SG ~2.4), and often shows flow banding or irregular bubbles. Mold seams and perfectly uniform round bubbles indicate manmade glass.
What is the difference between black obsidian and black glass?
Both are glass, so this is the hardest call. Natural obsidian usually shows flow banding and irregular, stretched bubble trails and never has mold seams. Manufactured 'slag' glass often has evenly spaced round bubbles, very uniform color, and sometimes seams from a mold.
Black obsidian vs black jasper — how do I tell them apart?
Obsidian is amorphous glass: softer (Mohs ~5), lighter, with a bright glassy luster and translucent edges. Black jasper is crystalline quartz: harder (7), denser, opaque, with a duller natural surface. A quartz point will scratch obsidian but not jasper.
Is obsidian a rock or a mineral?
Obsidian is technically a mineraloid — a natural volcanic glass. Because it is amorphous and lacks a crystal structure, it is not a true mineral, and because it is essentially homogeneous glass it is usually described as volcanic glass rather than a rock.
Black Obsidian identified by the community
Recent Black Obsidian specimens identified with Rock Identifier.
