Basalt Porphyry

Physical Properties

Hardness: 6-7 (Mohs), Color: Dark olive green to grey with black phenocrysts, Luster: Dull to sub-vitreous, Structure: Aphanitic matrix with visible crystals, Specific Gravity: 2.8-3.0

Formation & Geological History

Formed from volcanic activity where magma cooled in two stages: slowly underground to form large crystals (phenocrysts), then rapidly at the surface to form the fine-grained matrix. Often associated with Cenozoic volcanic flows.

Uses & Applications

Used primarily in construction as crushed stone or aggregate. Tumbled specimens are used in decoration or lapidary art.

Geological Facts

The larger black crystals (phenocrysts) in these stones are often minerals like augite or hornblende. This specific texture is called 'porphyritic'.

Field Identification & Locations

Identify in the field by looking for a fine-grained dark base rock containing distinct, larger individual mineral crystals. Commonly found in riverbeds within volcanic regions.

Identified on: 4/30/2026

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