metamorphic
Dark gray metamorphic cobble, likely basalt or hornfels
Probable hornfels or fine-grained basalt; exact identification requires testing
AI-generated identification · may be incorrectFine-grained, dark gray, dull to faintly satiny surface with scattered pale mineral flecks and a weathered fracture. Likely hardness about 5–7 Mohs; no obvious cleavage or visible crystals, and probably dense with specific gravity near 2.8–3.0.
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Physical properties
Fine-grained, dark gray, dull to faintly satiny surface with scattered pale mineral flecks and a weathered fracture. Likely hardness about 5–7 Mohs; no obvious cleavage or visible crystals, and probably dense with specific gravity near 2.8–3.0.
Formation & geological history
If hornfels, it formed when sedimentary rock was baked by a nearby igneous intrusion, commonly during the Mesozoic or older. If basalt, it crystallized rapidly from mafic lava; the rounded shape indicates transport by water or glacial ice.
Uses & applications
Common material has little commercial value as an individual specimen, though basalt is used for aggregate, roadstone, riprap, and landscaping; hornfels is occasionally used as durable crushed stone.
Geological facts
The rounded, heavily weathered form is consistent with a river or glacial cobble. A streak, magnetism test, dilute acid test, and fresh-break examination would distinguish basalt from hornfels more reliably.
Field identification & locations
Look for a uniformly fine texture, high density, and possible weak magnetism for basalt; hornfels may show subtle banding or relict sedimentary layering. Common in volcanic terrains, metamorphic aureoles, river gravels, and glacial deposits.
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