Basalt

Physical Properties

Hardness: 6-7 on Mohs scale. Color: Black, dark gray, or greenish-black. Luster: Dull to sub-metallic. Crystal structure: Fine-grained (aphanitic), typically containing plagioclase feldspar, pyroxene, and olivine. 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. This specimen shows water-worn rounding, suggesting it was transported by a river or coastal waves.

Uses & Applications

Used extensively in construction as road base, railroad ballast, and aggregate in concrete. High-quality dense basalt is also used for floor tiles and monuments.

Geological Facts

Basalt is the most common rock type in the Earth's crust and makes up most of the ocean floor. Large areas of the moon's surface, known as 'maria,' are also composed of basaltic lava flows.

Field Identification & Locations

Identify in the field by its dark color, heavy weight (relative to size), and fine-grained texture that lacks visible large crystals. It often feels slightly gritty despite a smooth rounded shape from erosion.

Identified on: 4/19/2026

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