igneous
Weathered basalt fragment
Basalt, a fine-grained mafic volcanic rock composed mainly of plagioclase and pyroxene, with possible olivine and iron-oxide alteration
AI-generated identification · may be incorrectDark gray to brown, moderately dull to subvitreous luster, dense and fine-grained with no visible cleavage; hardness about 5–6 Mohs and specific gravity roughly 2.8–3.0. The rusty brown surfaces likely reflect weathering of iron-bearing minerals.
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Physical properties
Dark gray to brown, moderately dull to subvitreous luster, dense and fine-grained with no visible cleavage; hardness about 5–6 Mohs and specific gravity roughly 2.8–3.0. The rusty brown surfaces likely reflect weathering of iron-bearing minerals.
Formation & geological history
Forms when low-silica basaltic lava rapidly cools at or near Earth’s surface, commonly in lava flows, oceanic crust, and volcanic fields. Its age cannot be determined from the image and may range from recent to billions of years.
Uses & applications
Used extensively as crushed aggregate, road base, railroad ballast, riprap, and building stone; attractive dense pieces may be polished, though this fragment has little gem value.
Geological facts
Basalt is Earth’s most abundant volcanic rock and makes up much of the ocean floor. Vesicles, if present, may later fill with secondary minerals such as zeolite, calcite, or agate.
Field identification & locations
Identify by its dark color, fine texture, heavy feel, and lack of large visible crystals; a streak, magnetism test, and fresh broken surface help distinguish it from weathered sedimentary rocks. Common worldwide, especially volcanic regions and oceanic crust exposures.
