igneous
Rounded basalt pebble
Basalt; fine-grained mafic volcanic rock, chiefly plagioclase and pyroxene with minor olivine or magnetite
AI-generated identification · may be incorrectTypically dark gray to black, dull to faintly vitreous, with Mohs hardness about 5–6 and no visible cleavage; the smooth shape indicates water or abrasion. The pale vein or patch may be weathered feldspar, calcite, or quartz, so exact identification requires testing.
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Physical properties
Typically dark gray to black, dull to faintly vitreous, with Mohs hardness about 5–6 and no visible cleavage; the smooth shape indicates water or abrasion. The pale vein or patch may be weathered feldspar, calcite, or quartz, so exact identification requires testing.
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 photograph and may range from recent to billions of years.
Uses & applications
Commonly used as crushed aggregate, road stone, riprap, and paving material; attractive rounded pieces are sold mainly as inexpensive décor or teaching specimens.
Geological facts
Basalt is the most abundant volcanic rock and makes up much of the ocean floor. Vesicles, if present, may later fill with minerals such as calcite, zeolites, or quartz.
Field identification & locations
Look for a dense, dark, fine-grained surface, weak magnetism, and a streak that is generally gray rather than strongly colored; it should not fizz vigorously in dilute acid unless the pale area is carbonate. Common around volcanic regions and streambeds downstream from them.
