Obsidian
Silica-rich volcanic glass (SiO₂-dominant)
A glassy, jet-black volcanic rock formed when lava cools too fast to crystallize, prized for razor-sharp conchoidal edges.
- Mohs hardness
- 5-6
- Color
- Black, sometimes brown, mahogany, or sheen
- Type
- igneous
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Overview
Obsidian is a naturally occurring volcanic glass formed when silica-rich lava cools so rapidly that mineral crystals have no time to grow. The result is an amorphous, glassy rock—usually jet black and lustrous, sometimes brown, red (mahogany), or showing iridescent sheen.
Because it has no crystal structure, obsidian breaks with sharp conchoidal (shell-like) fractures that can be honed to edges sharper than surgical steel. Ancient cultures worldwide knapped it into blades, arrowheads, and tools.
Varieties include snowflake obsidian (with cristobalite spherulites), rainbow and sheen obsidian, and small rounded nodules called Apache tears.
Formation & geology
Obsidian forms at the margins of rhyolitic lava flows and volcanic domes, where felsic (high-silica) lava is too viscous and cools too quickly for crystals to nucleate. This rapid quenching, often where lava meets air or water, freezes the melt into glass.
Because glass is metastable, ancient obsidian gradually devitrifies (crystallizes) over millions of years, so geologically old obsidian is rare—most is from recent volcanism.
Notable sources include the western USA (Oregon, California, Yellowstone), Mexico, Iceland, Italy (Lipari), and Armenia.
How to identify it
- Appearance: Glassy, often glossy black; translucent on thin edges. Snowflake and sheen varieties have distinctive patterns.
- Fracture: Sharp conchoidal (curved, shell-like) fracture—a key diagnostic.
- Hardness: About 5-6; scratches with a steel file but is brittle.
- No crystals or grains: It is uniform glass, unlike grainy igneous rocks.
Look-alikes: Black glass slag (manufactured) can mimic it but often has bubbles and an unnaturally even color. Onyx and black tourmaline are crystalline and harder. Jet is much lighter (organic) and warmer to the touch.
Uses & significance
Archaeologically, obsidian was vital for making knives, scrapers, arrowheads, and mirrors because of its glass-sharp edges; it remains a model material for studying ancient trade routes.
Today it is cut into gemstones, beads, carvings, and decorative objects, with snowflake, mahogany, and rainbow varieties especially popular. Extremely thin obsidian blades have even been used experimentally as surgical scalpels.
In metaphysical traditions black obsidian is linked to protection and grounding—cultural beliefs, not scientific fact.
Frequently asked questions
Is obsidian a rock, mineral, or glass?
Obsidian is a volcanic rock made of natural glass. Because it lacks a crystalline structure, it is technically a mineraloid rather than a true mineral.
Why is obsidian so sharp?
Its glassy, non-crystalline structure breaks along smooth conchoidal fractures, producing edges only a few molecules thick—sharper than a steel scalpel.
Is all obsidian black?
Most is black, but trace elements and inclusions produce brown, mahogany, green, and rainbow/sheen varieties, plus white-spotted snowflake obsidian.
How do you tell obsidian from black glass?
Natural obsidian may contain tiny crystals, flow banding, or snowflake spherulites, while manufactured glass slag often shows abundant round bubbles and uniform color.
Obsidian guides
In-depth guides for identifying, valuing, and understanding Obsidian.
Obsidian identified by the community
Real specimens identified with Rock Identifier.
