igneous
Polished volcanic rock, likely basalt
Basalt; predominantly plagioclase feldspar, pyroxene, and commonly olivine
AI-generated identification · may be incorrectTypically dark gray to black, fine-grained, with a dull to slightly vitreous luster; Mohs hardness about 5–6, dense, and lacking visible cleavage. The glossy surface appears polished or naturally water-worn, with pale weathering or mineral crust.
Identified More igneous →
Explore Polished volcanic rock, likely basalt in the encyclopedia →Identify your own rocks.
Get a report just like this from any photo, free.
Physical properties
Typically dark gray to black, fine-grained, with a dull to slightly vitreous luster; Mohs hardness about 5–6, dense, and lacking visible cleavage. The glossy surface appears polished or naturally water-worn, with pale weathering or mineral crust.
Formation & geological history
Basalt forms when iron- and magnesium-rich lava rapidly cools at or near Earth’s surface, commonly from volcanic eruptions and oceanic crust formation. Most examples range from Holocene to Precambrian in age, depending on locality.
Uses & applications
Used as crushed aggregate, roadstone, building stone, paving, and occasionally as a polished lapidary specimen. It has limited jewelry value compared with gemstones.
Geological facts
Its dark color reflects abundant ferromagnesian minerals and low silica content. Vesicles, if present, may later fill with zeolite, calcite, or quartz.
Field identification & locations
Identify it by its black, heavy, fine-grained appearance and hardness; a streak test is usually gray to black, while acid produces no reaction. Common worldwide, especially in volcanic regions and river gravels.
