The sand grains in sedimentary rocks such as sandstone come from many different rocks and minerals and may have a variety of shapes and colors. Worldwide, the largest fractions of sand grains consist of quartz and feldspar, because these two silicates are among the most common minerals in Earth's crust. Quartz has the advantages of hardness and chemical stability; some quartz sand grains survive long enough to be recycled from old sandstones. Other silicate minerals, such as plagioclase and pyroxene, may also be found in sandstone. Geologists name varieties of sandstone based on their grain content: for example, arkose is a dark-colored, high-feldspar sandstone and the principal material in the Australian monolith known as Uluru, or Ayers Rock.