sandstones are medium-grained rocks composed of rounded or angular fragments of sand size, that often but not always have a cement uniting them together. these sand-size particles are often quartz but there are a few common categories and a wide variety of classification schemes that classify sandstones based on composition. classification schemes vary widely, but most geologists have adopted the dott scheme, which uses the relative abundance of quartz, feldspar, and lithic framework grains and the abundance of muddy matrix between these larger grains.