Andesite often weathers to various shades of brown. In the field, weathered specimens must be broken to properly see their color and their mineral composition. Classroom specimens usually do not require breakage.

Andesite is rich in plagioclase feldspar and amphibole minerals. Quartz and pyroxene minerals may be absent or present in small quantities. Small amounts of mica will be present as biotite or muscovite. Andesite usually does not contain olivine. The accompanying chart titled “Generalized Composition Ranges of Common Igneous Rocks” illustrates the mineral composition of andesite.