Porphyritic Basalt (River Rock)

Physical Properties

Hardness: 6-7 Mohs; Color: Dark grey to black matrix with white or light-colored phenocrysts; Luster: Dull to sub-vitreous; Structure: Porphyritic (fine-grained matrix with larger visible crystals); Specific Gravity: 2.8-3.0

Formation & Geological History

Formed from the rapid cooling of magnesium-rich and iron-rich lava at or near the Earth's surface. The larger crystals (phenocrysts) formed slowly underground before the lava erupted. This specific specimen has been rounded by fluvial or glacial transport.

Uses & Applications

Used primarily in construction as crushed stone for road base, concrete aggregate, and railroad ballast. Smooth river-worn specimens are popular in landscaping and decorative masonry.

Geological Facts

Basalt is the most common rock type in the Earth's crust and makes up most of the ocean floor. The white spots in this rock are likely plagioclase feldspar crystals that grew before the rest of the lava solidified.

Field Identification & Locations

Identify by the contrast between the dark, fine-grained groundmass and the distinct, lighter rectangular or lath-shaped crystals. Commonly found in volcanic regions or in riverbeds/beaches downstream from volcanic sources.

Identified on: 5/3/2026

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