igneous

Andesite

Andesite (Intermediate Extrusive Volcanic Rock)

Hardness: 6-7 (Mohs scale); Color: Gray to grayish-black; Luster: Dull or sub-vitreous; Crystal structure: Aphanitic (fine-grained) to porphyritic (larger crystals in fine matrix); Specific gravity: 2.4 - 2.8.

Hardness: 6-7 (Mohs scale)
Color: Gray to grayish-black
Luster: Dull or sub-vitreous

Identified February 19, 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 by the rapid cooling of intermediate lava at the Earth's surface, typically at subduction zones (convergent plate boundaries). These rocks can date from the Archean to the Holocene.

Uses & applications

Used primarily in the construction industry as crushed stone for road base, concrete aggregate, and railroad ballast. Occasionally used for decorative masonry or tiles.

Geological facts

Andesite is the second most common volcanic rock in Earth's crust after basalt. It is named after the Andes Mountains where it is found in abundance. The 1980 eruption of Mount St. Helens produced significant amounts of andesitic material.

Field identification & locations

Identify in the field by its medium gray color and the presence of small, visible needle-like crystals (plagioclase or hornblende) within a fine-grained matrix. It is commonly found in volcanic arcs like the Cascades or the Andes. Collectors should look for porphyritic specimens with well-defined phenocrysts.

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