igneous

Andesite

Andesite (Intermediate Extrusive Igneous Rock)

Hardness: 6-7 on Mohs scale; Color: typically light to dark gray (also greenish or brownish); Luster: Subvitreous to dull; Crystal structure: Aphanitic (fine-grained) to porphyritic (larger crystals like plagioclase in a fine matrix); Specific gravity: 2.4 - 2.8

Hardness: 6-7 on Mohs scale
Color: typically light to dark gray (also greenish or brownish)
Luster: Subvitreous to dull

Identified March 22, 2026More igneous →

Identify your own rocks.

Get a report just like this from any photo, free.

App Store Google Play

Physical properties

Formation & geological history

Formed through the rapid cooling of intermediate lava at the surface. It is primarily associated with subduction zones at convergent plate boundaries. The specimen in the image is likely from the Quaternary period (approx. 7,700 years ago) related to the eruption of Mount Mazama.

Uses & applications

Used primarily in the construction industry as crushed stone for road base, concrete aggregate, and railroad ballast. Historically and in modern landscape architecture, it is used for masonry, wall stone, and decorative pavers.

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. The photo features Crater Lake, Oregon, which was formed by the collapse of an andesitic-dacitic volcano.

Field identification & locations

Identify in the field by its characteristic gray color, fine-grained texture, and lack of visible quartz compared to rhyolite. It often contains 'phenocrysts' (small visible white crystals of plagioclase). Found in volcanic arcs globally (Cascades, Andes, Japan).

Other igneous specimens

See all

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