igneous

Andesite

Intermediate Volcanic Rock (Andesite)

Hardness: 6-7 (Mohs scale), Color: light to dark gray/greenish-gray, Luster: dull or sub-vitreous, Crystal Structure: fine-grained (aphanitic) often with larger phenocrysts (porphyritic), Specific Gravity: 2.4-2.8

Identified March 1, 2026More igneous →

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Physical properties

Formation & geological history

Formed through the cooling of intermediate lava at volcanic arcs above subduction zones; primarily composed of plagioclase feldspar and pyroxene or hornblende.

Uses & applications

Used extensively in the construction industry as crushed stone for road base, concrete aggregate, and railroad ballast; occasionally used for tiles or decorative monuments.

Geological facts

Andesite is the second most common volcanic rock in the Earth's crust after basalt; it is named after the Andes Mountains where it is found in abundance.

Field identification & locations

Identify in the field by its intermediate gray color and typical occurrence in volcanic terrains; look for small visible crystals of feldspar or darker minerals embedded in a fine-grained matrix.

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