igneous
Tumbled reddish-brown basalt or andesite cobble
Fine-grained mafic to intermediate volcanic rock; commonly plagioclase, pyroxene, and magnetite, with iron-oxide staining
AI-generated identification · may be incorrectTypically Mohs 5–6, opaque, orange-brown to dark gray, and dull to subtly waxy from weathering or tumbling; fine-grained with no visible crystals, weak or absent cleavage, and specific gravity about 2.7–3.1.
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Physical properties
Typically Mohs 5–6, opaque, orange-brown to dark gray, and dull to subtly waxy from weathering or tumbling; fine-grained with no visible crystals, weak or absent cleavage, and specific gravity about 2.7–3.1.
Formation & geological history
Formed when lava cooled rapidly at or near Earth’s surface, likely millions to billions of years ago depending on source terrain; the brown color reflects oxidation and surface weathering. Exact identification requires a fresh surface or testing.
Uses & applications
Used as landscaping stone, aggregate, paving material, and decorative tumbled stone; this rounded specimen has modest collector value unless locality-provenanced.
Geological facts
The rounded shape is natural abrasion or human tumbling, while dark streaks may represent mafic minerals, weathering veins, or inclusions. It could also be a weathered metamorphic rock, such as altered basalt, rather than a definitive basalt.
Field identification & locations
Look for a fine, compact texture, heavy feel, dull luster, and no fizz with dilute acid; a streak, magnetism test, and fresh break can distinguish basalt from jasper or iron-rich sandstone. Common in volcanic regions and glacial- or stream-gravel deposits.
