A basic distinction can be drawn between seven broad groups of geological materials, although the boundaries between them are not rigid: rocks, sediments, soils, dusts, minerals, fossils, and particulates. Inorganic particles have formed without any significant involvement of biological processes; they mainly represent rock fragments, individual crystalline mineral grains or fragments of amorphous matter such as volcanic glass. Most are naturally formed, although some, such as concrete and brick fragments, may be anthropogenic. Organic particles are those that have formed directly as a result of biological processes. Igneous rocks are formed by crystallization from molten magma, either below Earth's surface or at the surface. Several different classification schemes for igneous rocks have been proposed on the basis of chemical composition, actual or normative mineralogical composition, and/or texture. Sediments are loose mixtures of particles, derived from one or more sources, which have been deposited after being transported by air, water, or ice.