River Stone (Basalt Porphyry)

Physical Properties

Hardness: 6-7 (Mohs scale); Color: Dark grey to black with light grey/white phenocrysts; Luster: Dull to waxy when polished; Crystal structure: Aphanitic matrix with visible phenocrysts; Cleavage: None/Irregular fracture.

Formation & Geological History

Formed from the rapid cooling of magnesium-rich and iron-rich lava at or near the Earth's surface. The large crystals (phenocrysts) formed earlier in the cooling process underground. This specific specimen has been rounded and smoothed by water erosion in a river or coastal environment.

Uses & Applications

Commonly used for landscaping, garden decoration, massage therapy (hot stones), and as a basic construction aggregate. Polished versions are used in home decor.

Geological Facts

Basalt is the most common rock type in the Earth's crust and makes up most of the ocean floor. The light line visible on the pebble is likely a small quartz or calcite vein that filled a fracture in the basalt.

Field Identification & Locations

Identify in the field by its dark, heavy nature and smooth, water-worn texture. It is frequently found in riverbeds, glacial deposits, and along rocky beaches.

Identified on: 4/22/2026

Mode: Standard