Basalt with Epidote (Porphyry)

Physical Properties

Hardness: 6-7 (Mohs). Color: Dark grey to greenish-black with light green or cream inclusions. Luster: Dull to sub-vitreous. Structure: Aphanitic (fine-grained) matrix with phenocrysts. 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 lighter inclusions often represent later hydrothermal alteration or phenocrysts formed deeper in the crust.

Uses & Applications

Used primarily in construction as crushed stone, road base, and concrete aggregate. Smooth river-worn specimens are popular for home garden decor and amateur rock collecting.

Geological Facts

Basalt is the most common rock type in the Earth's crust and makes up most of the ocean floor. The green inclusions (likely epidote) are often signs of low-grade metamorphism.

Field Identification & Locations

Identify in the field by its high density, dark color, and fine-grained texture. It is frequently found as rounded cobbles in riverbeds or on beaches near volcanic regions.

Identified on: 4/30/2026

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