As its name implies, sandstone is sand fused into rock. On account of their different manner of origin, the sandstones vary in mineral composition far more markedly than do the igneous rocks. Their primary mineral is mostly quartz, which is usually present as a binding agent between coarser mineral grains. Other minerals commonly found in addition to quartz are the feldspars (potassium feldspar or plagioclase) and calcite. Sandstones can be of many colours, e.g. yellow.