igneous

Porphyritic Andesite

Andesite (Porphyritic texture)

Hardness: 6-7 Mohs; Color: Gray to bluish-gray groundmass with white, cream, or pinkish phenocrysts; Luster: Dull to sub-vitreous; Crystal structure: Aphanitic (fine) background with coarse crystals; Cleavage: None (rock mass); Specific gravity: 2.4-2.8

Hardness: 6-7 Mohs
Luster: Dull to sub-vitreous

Identified May 25, 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-composition lava at or near the Earth's surface, typically in volcanic arc environments above subduction zones. The larger phenocrysts grew slowly underground before the final eruption.

Uses & applications

Used primarily as crushed stone for road construction, railroad ballast, and aggregate in concrete. It is also used as masonry stone and occasionally polished for architectural trim.

Geological facts

The name 'Andesite' is derived from the Andes Mountains in South America. The 'polka dot' look of this specimen is caused by large feldspar crystals cooling at a different rate than the surrounding volcanic matrix.

Field identification & locations

Identify by looking for distinct, larger 'spots' (phenocrysts) embedded in a fine-grained, solid gray background. It is commonly found in the Pacific Northwest, the Andes, and glaciated areas where it has been transported as till.

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