sandstone, sedimentary rock formed by the cementing together of grains of sand. The usual cementing material in sandstone is calcium carbonate, iron oxides, or silica, and the hardness of sandstone varies according to the character of the cementing material; quartz sandstones cemented with quartz are the hardest. Sandstones are commonly gray, buff, red, or brown although green and some other colors are also found. Green sandstones often contain, in addition to sand and glauconite, fossil shells and iron oxides; those that break apart easily are known as greensands and are sometimes used to replenish depleted potash in soils. Sandstones are widely used in construction and industry. Varieties of sandstone include arkose, which contains feldspar and resembles granite, and graywacke, a gray or sometimes greenish or black rock composed of quartz and fledspar with numerous fragments of other rocks, such as shale, slate, quartzite, granite, and basalt. Sandstone may be crushed to the form of loose sand grains, which can then be put to the same industrial uses as sand. See brownstone.
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