igneous
Porphyritic Basalt or Spotted Pebble
Porphyritic volcanic rock with phenocrysts
Hardness of 6-7 on the Mohs scale, colors ranging from dark gray to black with contrasting white or light-colored plagioclase phenocrysts (spots), dull to sub-to-submetallic luster, fine-grained aphanitic groundmass with distinct larger crystals.
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Physical properties
Hardness of 6-7 on the Mohs scale, colors ranging from dark gray to black with contrasting white or light-colored plagioclase phenocrysts (spots), dull to sub-to-submetallic luster, fine-grained aphanitic groundmass with distinct larger crystals.
Formation & geological history
Formed from cooling of magnesium- and iron-rich magma in volcanic or shallow intrusive settings, where slow cooling allowed larger crystals (phenocrysts) to form before rapid cooling solidified the groundmass.
Uses & applications
Used primarily in landscaping, decorative stone, road aggregate, and as a natural geological curiosity or specimen for collectors.
Geological facts
The distinct spotted pattern is caused by differential weathering or contrasting mineral inclusions like plagioclase feldspar crystals embedded in a finer dark matrix. Water tumbling often smooths and rounds these into distinctive beach or river pebbles.
Field identification & locations
Easily identified by its contrasting light spots against a dark background matrix. Commonly found in glacial drift, riverbeds, and beaches where volcanic rocks have been transported and smoothed.
