igneous
Polished porphyritic basalt (likely)
Basalt, a fine-grained mafic volcanic rock; visible inclusions may be olivine, feldspar, or vesicle infillings
AI-generated identification · may be incorrectDark brown-black, smooth polished surface with scattered rounded gray and pale inclusions; generally vitreous to dull luster, hardness about 5–6 Mohs, dense, with no obvious cleavage. The spots may be phenocrysts or amygdules rather than true holes.
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Physical properties
Dark brown-black, smooth polished surface with scattered rounded gray and pale inclusions; generally vitreous to dull luster, hardness about 5–6 Mohs, dense, with no obvious cleavage. The spots may be phenocrysts or amygdules rather than true holes.
Formation & geological history
Formed when mafic lava rapidly cooled at or near Earth’s surface, commonly during Cenozoic or older volcanic activity; the exact age and locality cannot be determined from the image.
Uses & applications
Used widely as crushed aggregate, road stone, building stone, and decorative polished material; this small piece has modest lapidary or teaching value.
Geological facts
Basalt is the most common volcanic rock on Earth and makes up much of the oceanic crust. Polishing can reveal contrasting mineral grains and filled gas cavities.
Field identification & locations
Check for a very fine dark matrix, high density, weak magnetism, and hardness sufficient to scratch glass; acid should not fizz unless carbonate-filled cavities are present. Common worldwide, especially volcanic regions.
