igneous
Weathered basalt (possibly iron-stained volcanic rock)
Basalt; primarily plagioclase feldspar and pyroxene, commonly with olivine and iron oxides
AI-generated identification · may be incorrectTypically fine-grained, dark gray to black, with dull to subvitreous luster and hardness about 5–6 Mohs; this specimen has yellow-brown surface staining, likely limonite or altered volcanic minerals. Crystal structure is mostly microscopic, so cleavage is not obvious; specific gravity is commonly 2.8–3.0.
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Physical properties
Typically fine-grained, dark gray to black, with dull to subvitreous luster and hardness about 5–6 Mohs; this specimen has yellow-brown surface staining, likely limonite or altered volcanic minerals. Crystal structure is mostly microscopic, so cleavage is not obvious; specific gravity is commonly 2.8–3.0.
Formation & geological history
Basalt forms when mafic lava cools rapidly at or near Earth’s surface, commonly in oceanic crust, lava flows, and volcanic islands. Individual pieces may be geologically young or millions to billions of years old; age cannot be determined from the image.
Uses & applications
Used extensively as crushed aggregate, road base, railroad ballast, building stone, and occasionally polished decorative stone. Common unexceptional specimens have little commercial or jewelry value.
Geological facts
The yellow-brown patches may represent weathering of iron-bearing minerals rather than native gold. Basalt can contain small vesicles, olivine crystals, or magnetic iron oxides.
Field identification & locations
Identify it by its heavy, dark, fine-grained appearance, lack of visible layering, and possible weak magnetism; a streak, magnet, and fresh broken surface would help confirm it. Common worldwide, especially near volcanic regions and oceanic crust.
