Basalt with Caliche Crust

Physical Properties

Hardness: 5-6 (interior) / 3 (coating); Color: dark grey to black interior, tan/buff coating; Luster: Dull/Earthy; Structure: Fine-grained aphanitic; Cleavage: None

Formation & Geological History

Formed from the rapid cooling of magnesium-rich and iron-rich lava at or very near the Earth's surface. The tan surface layer is 'caliche,' a sedimentary mineral deposit formed through the evaporation of mineral-rich water in arid environments.

Uses & Applications

Common basalt is used in construction as aggregate, road base, and railroad ballast. This specific weathered specimen is primarily of interest for geological study or casual rock collecting.

Geological Facts

Basalt is the most common rock type in the Earth's crust and makes up most of the ocean floor. Caliche coatings (the tan patches) often form in desert soils where calcium carbonate precipitates out of groundwater.

Field Identification & Locations

Identify by its heavy weight (high density), dark fine-grained interior, and the tan crust that will fizz if exposed to a drop of white vinegar. Commonly found in volcanic fields and arid alluvial plains.

Identified on: 4/15/2026

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