Sandstone. Credit: Mineral Information Institute.
Sandstone is variously colored sedimentary rock composed mainly of sandlike quartz grains cemented by calcite (calcareous sandstone), clay, or iron oxide (ferruginous sandstone). The sand accumulated originally underwater in shallow seas or lakes, or on the ground along shorelines or in desert regions. The rounded quartz grains are 0.06–2 millimeters across.
The chief varieties of sandstone are quartzite, rich in silica; arkose, feldspar-rich; graywacke, coarse-grained and of varied composition; and subgraywacke, with more rounded grains and less feldspar than graywacke. Sandstone grades into conglomerate and shale.