igneous
Tumbled black stone, likely basalt
Basalt, chiefly plagioclase and pyroxene; exact identification requires testing
AI-generated identification · may be incorrectFine-grained, opaque dark gray-black, dull to subtly glassy luster, rounded by tumbling; hardness about 5–6 Mohs, no visible cleavage, specific gravity roughly 2.8–3.1. Small pits and pale surface patches are consistent with vesicles or weathering.
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Physical properties
Fine-grained, opaque dark gray-black, dull to subtly glassy luster, rounded by tumbling; hardness about 5–6 Mohs, no visible cleavage, specific gravity roughly 2.8–3.1. Small pits and pale surface patches are consistent with vesicles or weathering.
Formation & geological history
Basalt forms when low-silica basaltic lava cools rapidly at or near Earth’s surface, commonly in volcanic fields and oceanic crust; specimens may range from recent to billions of years old. The smooth shape is human-produced by abrasion.
Uses & applications
Used as aggregate, road stone, riprap, and dimension stone; attractive polished pieces are sold for crafts, massage stones, and inexpensive collections. It has little gemstone value unless unusually fine-grained or marketed as a specialty stone.
Geological facts
The image alone cannot distinguish basalt confidently from black chert, gabbro, slag, or another dark stone. A magnet test may show weak attraction if iron-rich minerals are present, but this is not diagnostic.
Field identification & locations
Basalt typically has a fine texture and is heavier than glass or many plastics; a streak test, hardness test, acid reaction, and close examination for glassy or vesicular texture can help. Common sources include volcanic regions worldwide.
