Basalt with Plagioclase Microphenocrysts

Physical Properties

Hardness: 6 (Mohs); Color: Dark grey to black matrix with white/off-white inclusions; Luster: Dull to sub-vitreous; Crystal structure: Aphanitic (fine-grained) with larger macro-crystals; SG: 2.8–3.0

Formation & Geological History

Formed from the rapid cooling of magnesium-rich and iron-rich lava exposed at or very near the surface. The white patches represent secondary mineral infilling or localized zones of plagioclase feldspar crystallization.

Uses & Applications

Mainly used as aggregate in construction, road base, and railroad ballast. Larger specimens are of interest to student geologists for Studying volcanic cooling rates.

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 and Mars.

Field Identification & Locations

Identify in the field by its heavy weight (high density), dark matrix, and fine-grained texture. Commonly found in volcanic fields, oceanic islands, and mid-ocean ridges.

Identified on: 5/20/2026

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