Basalt

Physical Properties

Hardness: 6 (Mohs scale); Color: Dark grey to black; Luster: Dull/earthy; Crystal structure: Aphanitic (fine-grained); Presence of small vesicles (gas bubbles); Significant density and toughness.

Formation & Geological History

Formed from the rapid cooling of magnesium-rich and iron-rich lava at the earth's surface. This specific specimen shows weathering/rounding from water transport, likely in a river or coastal environment.

Uses & Applications

Commonly used in construction as crushed stone for road base, concrete aggregate, and railroad ballast. Small rounded specimens like this are often used in landscaping or as decorative 'river rocks'.

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. The 'holes' seen in this specimen are called vesicles, formed by gas escaping from cooling lava.

Field Identification & Locations

Identify in the field by its dark color, heavy weight (high specific gravity), and fine-grained texture. It often weathers to a lighter brown or green over time. This specimen is easily found in volcanic regions or in commercial landscaping pebble mixes.

Identified on: 4/24/2026

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