igneous
Tumbled dark gray stone, likely basalt or fine-grained volcanic rock
Basalt (tentative); composed mainly of plagioclase feldspar, pyroxene, and possibly olivine
AI-generated identification · may be incorrectVery fine-grained, rounded and polished, dark gray with subtle mottling and a dull-to-satin luster; typically hardness 5–6, no visible cleavage, and specific gravity about 2.8–3.1. The photograph does not show enough detail for confident species-level identification.
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Physical properties
Very fine-grained, rounded and polished, dark gray with subtle mottling and a dull-to-satin luster; typically hardness 5–6, no visible cleavage, and specific gravity about 2.8–3.1. The photograph does not show enough detail for confident species-level identification.
Formation & geological history
Basalt forms when low-silica lava cools rapidly at or near Earth’s surface, commonly in oceanic crust, volcanic fields, and flood-basalt provinces. Specimen age depends on its source and may range from recent to billions of years.
Uses & applications
Used as crushed aggregate, road stone, building stone, ballast, and occasionally in landscaping or decorative tumbled-stone collections. It has little jewelry value compared with gemstones.
Geological facts
Basalt is the most abundant volcanic rock and makes up much of the ocean floor. A magnet or streak test may help if the specimen contains abundant iron-bearing minerals.
Field identification & locations
Look for a uniformly fine texture, dark color, high density, and absence of large crystals; a glassy surface, strong magnetism, or vesicles can provide additional clues. Common sources include Hawaii, Iceland, India, the western United States, and oceanic islands.
