Classification of andesites may be refined according to the most abundant phenocryst. Biotite is an essential phenocryst in some varieties of lamprophyre. This is why most igneous rocks have only one type of phenocryst mineral. A similar type of crystal is a "phenocryst", a large crystal in an igneous rock. The usual phenocryst habit is the ones commonly observed. Olivine is the most important phenocryst, although clinopyroxene and spinel are also found. Phenocryst phases include amphibole, biotite, plagioclase, pyroxene and titanomagnetite. Phenocryst phases contained in the rocks are olivine, plagioclase and pyroxene. The most important criterion is the phenocryst species, followed by the groundmass mineralogy. Olivine may also be a phenocryst, and when present, may have rims of pigeonite.