igneous
Tumbled basalt pebble
Basalt; primarily plagioclase feldspar, pyroxene, and commonly olivine
AI-generated identification · may be incorrectFine-grained, dark brown to black, with a dull to faintly glassy luster; hardness about 5–6 Mohs, no visible cleavage, and typical specific gravity around 2.8–3.0. The smooth rounded shape indicates natural water wear or tumbling.
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Physical properties
Fine-grained, dark brown to black, with a dull to faintly glassy luster; hardness about 5–6 Mohs, no visible cleavage, and typical specific gravity around 2.8–3.0. The smooth rounded shape indicates natural water wear or tumbling.
Formation & geological history
Basalt forms when low-silica mafic lava cools rapidly at or near Earth’s surface, commonly in volcanic fields, oceanic crust, and flood-basalt provinces. Its age cannot be determined from the image and may range from recent to billions of years.
Uses & applications
Used as aggregate, road material, riprap, ballast, dimension stone, and landscaping rock; polished pieces are inexpensive decorative specimens. It is not normally used as a gemstone.
Geological facts
The specimen appears compact and vesicle-free, though surface texture and lighting obscure details; a slight reddish-brown tone may reflect weathering or iron oxidation. Basalt is the most abundant volcanic rock on ocean floors.
Field identification & locations
Identify by its dark, dense, fine-grained appearance, lack of obvious crystals, and resistance to scratching; it should not scratch easily with a steel knife unless weathered. Common sources include volcanic regions worldwide, beaches, river gravels, and commercial tumbling mixes.
