igneous

Andesite

Andesite (Intermediate Volcanic Rock)

Hardness: 6-7 (Mohs scale); Color: medium gray to grayish-green; Luster: dull to earthy; Structure: fine-grained (aphanitic) sometimes with porphyritic crystals of plagioclase fieldspar or pyroxene; Specific Gravity: 2.4 - 2.8

Hardness: 6-7 (Mohs scale)
Color: medium gray to grayish-green
Luster: dull to earthy

Identified May 6, 2026More igneous →

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Physical properties

Formation & geological history

Formed through the rapid cooling of lava at the Earth's surface, typically at convergent plate boundaries (subduction zones). It is associated with volcanic arc activity and dates from various geological eras including the Phanerozoic.

Uses & applications

Used primarily in the construction industry as crushed stone for road bases, concrete aggregate, and railroad ballast. Exceptionally durable varieties are sometimes used for landscaping or as dimensional stone.

Geological facts

Andesite was named after the Andes Mountains in South America, where this rock type is incredibly abundant. It bridges the chemical gap between basalt and dacite/rhyolite.

Field identification & locations

Identify in the field by its medium gray color and fine-grained texture that is lighter than basalt but darker than rhyolite. Commonly found in the Andes, Cascade Range of North America, and Japan. Best identified by examining fresh breaks for small, lath-like plagioclase crystals.

Other igneous specimens

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Blue Apatite

Fluorapatite (Ca5(PO4)3F)

mineral

Dark Igneous Rock (Likely Basalt or Diabase)

Basalt (extrusive igneous rock) or Diabase (intrusive igneous rock - also known as Dolerite), largely composed of plagioclase feldspar, pyroxene (augite), and sometimes olivine and amphibole. Exact mineralogy would require thin section analysis.

Igneous