igneous
Porphyry (Basaltic / Andesitic Porphyry)
Porphyritic Basalt / Porphyritic Andesite
Hardness 6-7 on Mohs scale; greenish-grey matrix with lighter rounded to sub-angular phenocrysts (plagioclase feldspar); dull to waxy luster; aphantic matrix with porphyritic texture.
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Physical properties
Hardness 6-7 on Mohs scale; greenish-grey matrix with lighter rounded to sub-angular phenocrysts (plagioclase feldspar); dull to waxy luster; aphantic matrix with porphyritic texture.
Formation & geological history
Formed via two-stage cooling of volcanic magma. Phenocrysts crystallized slowly at depth, followed by rapid cooling of the remaining melt closer to or at the surface. Commonly found in ancient volcanic terranes like the Great Lakes region.
Uses & applications
Used primarily as decorative pebbles, lapidary materials, landscape stone, and as aggregate for construction.
Geological facts
Porphyritic rocks are known as 'flower stones' or 'snowflake stones' when light-colored phenocrysts form distinct starburst or flower-like patterns in a dark matrix.
Field identification & locations
Look for distinct, lighter-colored mineral crystals suspended in a fine-grained, darker groundmass. Frequently smooth and rounded when retrieved from beach or river environments.
