igneous
Weathered volcanic rock, likely andesite or basalt
Fine-grained mafic to intermediate volcanic rock; probable andesite–basalt, compositionally dominated by plagioclase with pyroxene and possibly alteration minerals
AI-generated identification · may be incorrectGray-brown, dull to slightly earthy luster, very fine-grained and strongly weathered; likely Mohs 5–6, massive, with no obvious cleavage or visible crystals. Rounded shape reflects abrasion and weathering rather than crystal form.
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Physical properties
Gray-brown, dull to slightly earthy luster, very fine-grained and strongly weathered; likely Mohs 5–6, massive, with no obvious cleavage or visible crystals. Rounded shape reflects abrasion and weathering rather than crystal form.
Formation & geological history
Formed when lava cooled rapidly at or near Earth’s surface, generally during a volcanic episode; its exact geological age cannot be determined from the photograph. Surface oxidation and chemical weathering produced the brown coating.
Uses & applications
Commonly used as aggregate, road stone, riprap, and landscaping material; ordinary hand specimens have little commercial or jewelry value.
Geological facts
The specimen appears dense and compact, without visible vesicles, layering, fossils, or metallic material. Its rounded form may indicate stream, beach, or glacial transport.
Field identification & locations
Test for a dull streak, hardness sufficient to scratch glass, and a weak or absent reaction to dilute hydrochloric acid; a fresh broken surface would reveal texture and color more reliably. Common worldwide in volcanic terrains and river gravels.
