igneous
Tumbled dark stone, likely basalt or gabbro
Basalt (fine-grained mafic rock), possibly gabbro; mainly plagioclase and pyroxene
AI-generated identification · may be incorrectDark gray to black, smooth and rounded from tumbling, with a dull to slightly waxy luster; typically hardness 5–6 Mohs, massive texture, no visible cleavage, and specific gravity about 2.8–3.1.
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Physical properties
Dark gray to black, smooth and rounded from tumbling, with a dull to slightly waxy luster; typically hardness 5–6 Mohs, massive texture, no visible cleavage, and specific gravity about 2.8–3.1.
Formation & geological history
Forms from cooling iron- and magnesium-rich magma; basalt commonly develops in volcanic flows, while gabbro crystallizes slowly underground. Most examples range from recent to hundreds of millions of years old, depending on source.
Uses & applications
Used as aggregate, road stone, riprap, and landscaping material; attractive pieces are sold as inexpensive tumbled stones, worry stones, or craft material.
Geological facts
The specimen is too small and visually blurred for definitive identification; similar dark tumbled materials include andesite, slate, black chert, and industrial slag.
Field identification & locations
Look for a uniformly dark, fine-grained interior and a relatively high density; a streak test, magnetism test, and fresh unpolished surface can help distinguish basalt from slate or slag. Common worldwide, especially near volcanic regions and river gravels.
