igneous
Tumbled basalt pebble
Basalt; fine-grained mafic volcanic rock, chiefly plagioclase and pyroxene with possible olivine
AI-generated identification · may be incorrectTypically Mohs 5–6, dark gray to brown-black, dull to subtly vitreous, and very fine-grained; the smooth rounded surface indicates natural water wear or tumbling. Crystal structure is microscopic, with no visible cleavage.
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Physical properties
Typically Mohs 5–6, dark gray to brown-black, dull to subtly vitreous, and very fine-grained; the smooth rounded surface indicates natural water wear or tumbling. Crystal structure is microscopic, with no visible cleavage.
Formation & geological history
Basalt forms when iron- and magnesium-rich lava rapidly cools at or near Earth’s surface, commonly in oceanic crust, volcanic fields, and flood-basalt provinces. Its age cannot be determined from the image and may range from modern to billions of years.
Uses & applications
Used as aggregate, roadstone, construction stone, ballast, and landscaping material; polished pieces are inexpensive decorative or worry stones rather than jewelry gemstones.
Geological facts
The brown-purple coloration may reflect surface weathering, oxidation, or lighting. Basalt is denser and generally darker than granite, with little or no visible quartz.
Field identification & locations
Field clues include high density, uniform fine texture, dark color, and possible tiny vesicles; a streak, magnetism test, and fresh broken surface would improve identification. Common worldwide, especially volcanic regions and river beaches.
