Basalt

Physical Properties

Hardness: 5-6 Mohs. Color: dark gray, black, or greenish-black. Luster: dull to earthy. Texture: fine-grained (aphanitic) with small holes called vesicles. Crystal structure: microcrystalline to glassy.

Formation & Geological History

Formed from the rapid cooling of magnesium-rich and iron-rich lava (mafic lava) at or very near the Earth's surface. The holes (vesicles) were formed by gas bubbles trapped in the lava during solidification.

Uses & Applications

Used extensively in construction for road base, concrete aggregate, and railroad ballast. Also used as decorative landscaping stone and occasionally in jewelry for 'lava rock' beads.

Geological Facts

Basalt is the most common rock in Earth's crust and makes up the majority of the ocean floor. It is cũng the primary rock found on the Moon (the dark lunar maria) and Mars.

Field Identification & Locations

Identify in the field by its dark color, heavy weight (high density), and fine-grained texture. Commonly found in volcanic regions like Iceland, Hawaii, and the Pacific Northwest.

Identified on: 4/16/2026

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