igneous

Andesite

Andesite (Intermediate Extrusive Volcanic Rock)

Hardness: 6-7 (Mohs scale); Color: light to dark grey, sometimes reddish-brown; Luster: dull/earthy; Crystal Structure: Aphanitic (fine-grained) often with visible phenocrysts of plagioclase or hornblende; Specific gravity: 2.4 - 2.8

Hardness: 6-7 (Mohs scale)
Color: light to dark grey, sometimes reddish-brown
Luster: dull/earthy

Identified March 29, 2026More igneous →

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Physical properties

Formation & geological history

Formed through the rapid cooling of intermediate-composition lava at or near the surface, typically at volcanic arches above subduction zones. Can range from Cenozoic to ancient Precambrian ages depending on the tectonic history of the region.

Uses & applications

Primarily used in the construction industry as crushed stone for road base, concrete aggregate, and railroad ballast. Occasionally used for landscaping, pavers, or as a dimension stone for architectural cladding.

Geological facts

Andesite is named after the Andes Mountains in South America, where it is found in great abundance. It represents the chemical intermediate between basalt and dacite/rhyolite. Some of the world's most famous volcanoes, like Mount St. Helens, are primarily composed of andesite.

Field identification & locations

Identify in the field by its medium-grey color (lighter than basalt), fine-grained texture, and lack of visible quartz crystals. Common in volcanic regions of Western North America, the Andes, Japan, and Indonesia. Look for 'porphyritic' textures where larger crystals are embedded in a fine-grey 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

Magnetite

Iron(II,III) oxide (Fe3O4)

mineral

Sodalite

Sodalite - Na8(Al6Si6O24)Cl2

mineral

Blue Apatite

Apatite (Group), Ca5(PO4)3(F,Cl,OH)

mineral

Magnetite

Magnetite (Fe3O4)

mineral