Basalt with Plagioclase Phenocrysts (Porphyritic Basalt)

Physical Properties

Hardness: 6-7 on Mohs scale. Color: Dark grey to charcoal matrix with pale white/grey inclusions. Luster: Dull/earthy (weathered) to vitreous. Structure: Porphyritic (large crystals in a fine-grained matrix). 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 large white spots (phenocrysts) formed slowly underground before the lava erupted, while the dark matrix cooled quickly after eruption.

Uses & Applications

Used in construction as crushed stone, floor tiles, and garden landscaping. High-quality basalt is used in stone wool insulation or as 'hot stones' for massage therapy.

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 extensively on the Moon and Mars.

Field Identification & Locations

Identify in the field by its heavy weight, dark color, and visible rectangular or rounded white mineral spots. Commonly found in volcanic regions, riverbeds, and glacial deposits.

Identified on: 4/24/2026

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