Sandstone (sometimes known as arenite) is a clastic sedimentary rock composed mainly of sand-sized minerals or rock grains cemented together. Most sandstone is composed of quartz and/or feldspar because these are the most common minerals in the Earth’s crust. Like sand, sandstone may be any color, but the most common colors are: tan, brown, yellow, red, grey, pink, white, and black. Sandstone beds often form highly visible cliffs and other prominent topographic features. Sandstones of certain colors are often associated with specific regions. Rock formations that are primarily composed of sandstone usually allow percolation of water and other fluids and are porous enough to store large quantities, making them valuable aquifers and petroleum reservoirs. Quartz-bearing sandstone is converted into quartzite through heating and pressure usually related to tectonic compression within orogenic belts. Sandstones are clastic in origin (as opposed to either organic - like chalk and coal, or chemical - like gypsum and halite). They are formed from cemented grains that may either be fragments of a pre-existing rock or be monomineralic crystals. The cements binding these grains together are typically calcite, clays, and silica. Grain sizes in sands are defined (in geology) within the range of 0.0625 mm to 2 mm. Clays and sediments with smaller grain sizes not visible with the naked eye, including siltstones and shales, are called argillaceous. Rocks with greater grain sizes, including breccias and conglomerates, are termed rudaceous. The most common cementing materials are silica and calcium carbonate, which are often derived either from dissolution or from alteration of the sand after it was buried.