Black Obsidian
Volcanic glass, approx. 70-75% SiO2 (amorphous)
Jet-black natural volcanic glass formed by rapidly cooled lava, prized for its glassy luster and razor-sharp conchoidal fracture.
- Mohs hardness
- 5-5.5
- Color
- Deep black, sometimes with brown or gray tones
- Type
- igneous
Got a rock like this?
Identify any rock from a photo, free.
Overview
Black obsidian is a naturally occurring volcanic glass formed when silica-rich lava cools so quickly that mineral crystals have no time to grow. The result is an amorphous, non-crystalline solid with a smooth, glassy texture.
Its deep black color comes from finely dispersed iron and magnesium and microscopic mineral inclusions. Because it is glass, obsidian breaks with a smooth, curved conchoidal fracture that produces extremely sharp edges.
Obsidian has been used by humans for tens of thousands of years for cutting tools, weapons and mirrors. It is the most common and classic of the many obsidian varieties.
Formation & geology
Obsidian forms at the margins of rhyolitic lava flows and domes, where viscous, high-silica lava chills almost instantly on contact with air or water. The rapid quenching prevents atoms from arranging into crystals, locking them into a glassy state.
Because it is metastable, obsidian slowly devitrifies (crystallizes) over geologic time, so most obsidian is geologically young, typically less than about 20 million years old. Older glass has usually converted to fine-grained crystalline rock.
Major sources include volcanic regions of the western United States (Oregon, California, Arizona), Mexico, Iceland, Italy (Lipari), Turkey, Japan and other areas with explosive silicic volcanism.
How to identify it
Look for a glossy, glass-like black material with a vitreous luster and a smooth conchoidal (shell-like, curved) fracture that yields sharp edges. Obsidian is hardness 5-5.5, will scratch glass slightly, and feels noticeably warm and light compared to most rocks.
It is translucent on thin edges, often showing brown or smoky tones against light, and may contain gas bubbles or tiny crystallites. It produces a white to colorless streak.
Look-alikes include black glass slag, jet, onyx and dark volcanic rock. Jet is much lighter and warmer to touch; onyx is crystalline and harder; manufactured slag glass often has unnatural bubbles and color. The conchoidal fracture and glassy feel are key.
Uses & significance
Historically, black obsidian was knapped into knives, arrowheads, spear points and scrapers, and polished into mirrors; its edge can be sharper than steel, and it is still used in some specialty surgical scalpels.
Today it is widely cut and polished for beads, cabochons, spheres, carvings and decorative objects. It is inexpensive and easy to work, making it popular in jewelry and ornaments.
Metaphysically, black obsidian is associated with protection, grounding and "scrying" or self-reflection, though these claims are not scientific. Care is needed when handling raw pieces because broken edges are very sharp.
Frequently asked questions
Is obsidian a rock or a mineral?
It is a rock, specifically a natural volcanic glass. Because it is amorphous (non-crystalline), it is not classified as a true mineral.
Why is obsidian so sharp?
As a glass, it breaks along smooth conchoidal fractures that can create edges only a few molecules thick, sharper than a steel scalpel.
How can I tell real obsidian from black glass?
Natural obsidian often shows brown or smoky translucency on edges, tiny crystallites or bubbles, and a glassy conchoidal fracture; manufactured glass may have uniform color and regular bubbles.
Why is obsidian always geologically young?
Obsidian slowly crystallizes (devitrifies) over millions of years, so old volcanic glass converts to crystalline rock; most surviving obsidian is under about 20 million years old.
Is black obsidian safe to handle?
Polished pieces are safe, but freshly broken obsidian has extremely sharp edges and should be handled carefully.
Black Obsidian guides
In-depth guides for identifying, valuing, and understanding Black Obsidian.
Black Obsidian identified by the community
Real specimens identified with Rock Identifier.
