Any scheme for categorizing igneous rocks. The simplest classifications define rocks by their chemical and mineral composition, as felsic, intermediate, mafic, and ultramafic, and by their texture, as intrusive and coarse-grained, extrusive and fine-grained, or glass. In coarse-grained rocks the crystals are visible to the naked eye; in fine-grained rocks they are microscopic; and in glass there are no crystals (and therefore no minerals). The table shows a simple classification with examples.See also qapf classification.