Basalt (Porphyritic)

Physical Properties

Hardness: 6-7 on Mohs scale. Color: Dark grey to black with light-colored phenocrysts (specks). Luster: Dull to earthy. Structure: Fine-grained matrix (aphanitic) with larger embedded crystals. Specific Gravity: 2.8–3.0.

Formation & Geological History

Formed from the rapid cooling of magnesium-rich and iron-rich lava at or very near the Earth's surface. The 'porphyritic' texture indicates a two-stage cooling process where some crystals formed slowly underground before the rest of the magma erupted and cooled quickly.

Uses & Applications

Used extensively in construction as 'crushed stone' for road base, concrete aggregate, and railroad ballast. Also used as dimension stone for pavers and in some specialized industrial stone-wool insulation.

Geological Facts

Basalt is the most common rock type in the Earth's crust and makes up most of the ocean floor. It is also found on the Moon, Mars, and Venus.

Field Identification & Locations

Identify in the field by its dark, heavy nature and fine-grained texture. It often lacks the visible flakes of mica seen in darker metamorphic rocks. This specific piece shows visible white/tan crystals (phenocrysts) against a dark background.

Identified on: 4/15/2026

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