igneous
Weathered volcanic rock, likely basalt
Basalt, a fine-grained mafic igneous rock composed mainly of plagioclase feldspar and pyroxene, with possible olivine and alteration minerals
AI-generated identification · may be incorrectDark gray-brown, dull to slightly waxy luster, irregularly mottled and weathered; hardness about 5–6 Mohs, typically massive with no obvious cleavage, and specific gravity roughly 2.8–3.0. The rounded edges suggest natural abrasion or tumbling.
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Physical properties
Dark gray-brown, dull to slightly waxy luster, irregularly mottled and weathered; hardness about 5–6 Mohs, typically massive with no obvious cleavage, and specific gravity roughly 2.8–3.0. The rounded edges suggest natural abrasion or tumbling.
Formation & geological history
Formed when low-silica basaltic lava cooled rapidly at or near Earth’s surface; its exact geological age cannot be determined from the photograph. Surface weathering has likely produced the brown and tan patches.
Uses & applications
Common basalt is used as aggregate, road stone, riprap, concrete material, and occasionally dimension stone; this small specimen has little commercial value but may be suitable for a collection.
Geological facts
Basalt is Earth’s most abundant volcanic rock and forms much of the oceanic crust, including seafloor pillow lavas and large lava plateaus.
Field identification & locations
Look for a dense, fine-grained dark rock that does not fizz strongly in dilute hydrochloric acid and may attract a weak magnet if iron-rich. Common sources include volcanic regions, river gravels, beaches, and glacial deposits; exact identification would benefit from a fresh surface and magnification.
