Porphyritic Basalt

Physical Properties

Hardness: 6-7 (Mohs scale); Color: Dark grey to black groundmass with light-colored phenocrysts; Luster: Dull to sub-vitreous; Crystal Structure: Aphanitic (fine-grained) with larger visible phenocrysts (porphyritic); 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 at depth before the final eruption. Often found in volcanic island arcs and oceanic crust.

Uses & Applications

Used primarily as crushed stone for road construction, railroad ballast, and concrete aggregate. Select specimens are used in stone carving or as decorative landscaping rocks.

Geological Facts

This specimen appears to have 'amygdules,' which are secondary minerals that filled gas bubbles (vesicles) after the lava cooled. Basalt is the most common rock type in the Earth's crust and makes up most of the ocean floor.

Field Identification & Locations

Identify in the field by its dark, dense matrix and the presence of 'spots' (phenocrysts or amygdules). Common in volcanic regions like the Pacific Northwest, Hawaii, and Iceland.

Identified on: 5/3/2026

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