igneous
Possible obsidian or slag glass
Volcanic glass, primarily SiO₂; alternatively industrial iron-rich glassy slag
AI-generated identification · may be incorrectSmall black, glassy, irregular fragment with bright reflective surfaces and conchoidal-looking breaks. Obsidian has Mohs hardness about 5–5.5, no visible crystals, vitreous luster, and conchoidal fracture; the image is insufficient to confirm whether it is natural obsidian or man-made slag.
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Physical properties
Small black, glassy, irregular fragment with bright reflective surfaces and conchoidal-looking breaks. Obsidian has Mohs hardness about 5–5.5, no visible crystals, vitreous luster, and conchoidal fracture; the image is insufficient to confirm whether it is natural obsidian or man-made slag.
Formation & geological history
Obsidian forms when silica-rich lava cools rapidly, commonly during volcanic eruptions, usually in the Quaternary to older Cenozoic. Slag forms as a byproduct of metal smelting and is not a natural rock.
Uses & applications
Obsidian is used for decorative objects, tools, and occasionally jewelry; slag has little commercial value and is mainly collected as an industrial curiosity.
Geological facts
Obsidian can produce extremely sharp edges and was historically used for blades and arrowheads. A streak test, magnet test, and examination for bubbles or metallic flecks can help distinguish slag.
Field identification & locations
Common obsidian sources include Mexico, the western United States, Armenia, and Japan. Test for a white streak, lack of magnetism, and uniform glass; abundant bubbles, metal beads, or strong magnetism suggest slag.
