igneous
Basalt Porphyry
Porphyritic Basalt
Hardness: 6-7 (Mohs scale), Color: Dark grey to greenish-black matrix with light-colored phenocrysts (often plagioclase), Luster: Dull/Earthy (unless wet), Structure: Porphyritic (large crystals in a fine-grained groundmass)
Identified More igneous →
Explore Basalt Porphyry in the encyclopedia →Identify your own rocks.
Get a report just like this from any photo, free.
Physical properties
Hardness: 6-7 (Mohs scale), Color: Dark grey to greenish-black matrix with light-colored phenocrysts (often plagioclase), Luster: Dull/Earthy (unless wet), Structure: Porphyritic (large crystals in a fine-grained groundmass)
Formation & geological history
Formed from cooling lava that began crystallizing underground before erupting and cooling rapidly at the surface. Rounded by river or beach water erosion over thousands of years.
Uses & applications
Commonly used as construction aggregate, road base, and decorative river rock for landscaping. High-quality specimens are collected for educational purposes.
Geological facts
The 'spots' (phenocrysts) represent a two-stage cooling history: slow cooling deep underground to grow the large crystals, followed by rapid cooling during a volcanic eruption.
Field identification & locations
Identifiable by the distinct 'freckled' look of white crystals against a dark, dense rock. Look for them in areas with historic volcanic activity or in glacial till and riverbeds.
