igneous
Porphyritic Basalt (Porphyry pebble)
Porphyritic Basalt / Amygdaloidal Basalt
Hardness 6-7 on Mohs scale, dark greenish-brown fine-grained groundmass with light-colored (white to off-white) phenocrysts of plagioclase feldspar or quartz/calcite amygdules, vitreous to dull luster when wet.
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Physical properties
Hardness 6-7 on Mohs scale, dark greenish-brown fine-grained groundmass with light-colored (white to off-white) phenocrysts of plagioclase feldspar or quartz/calcite amygdules, vitreous to dull luster when wet.
Formation & geological history
Formed through two-stage cooling of magma: large crystals (phenocrysts) formed slowly underground, followed by rapid eruption and cooling of the surrounding lava fine-grained matrix. Often smoothed by wave action on coastal beaches.
Uses & applications
Used in landscaping, decorative stone collections, beach pebble crafts, and crushed stone for road construction.
Geological facts
The distinctive contrast between the large embedded white crystals and the dark fine-grained volcanic matrix gives porphyry a striking 'snowflake' or 'flower stone' pattern when polished.
Field identification & locations
Look for a fine-grained, dark green or grey volcanic rock matrix studded with distinct, larger white rectangular or rounded mineral inclusions. Commonly found on volcanic ocean beaches, such as along the Pacific Northwest coast.
