Graywacke (2.5 cm across at its widest)

Sedimentary rocks form by the solidification of loose sediments. Loose sediments become hard rocks by the processes of deposition, burial, compaction, dewatering, and cementation.

There are three categories of sedimentary rocks: 1) Siliciclastic sedimentary rocks form by the solidification of sediments produced by weathering & erosion of any previously existing rocks. 2) Biogenic sedimentary rocks form by the solidification of sediments that were once-living organisms (plants, animals, micro-organisms). 3) Chemical sedimentary rocks form by the solidification of sediments formed by inorganic chemical reactions. Most sedimentary rocks have a clastic texture, but some are crystalline.

Sandstones are the second-most common siliciclastic sedimentary rocks on Earth. They are composed of sand-sized grains - between 2 mm and 0.0625 mm in size. The vast majority are quartzose sandstones, which are dominated by quartz sand - SiO2. Exceptionally pure quartz sandstone is called quartz arenite. Other categories of sandstone include feldspathic sandstone (dominated by K-feldspar sand - KAlSi3O8) and lithic sandstone (dominated by sand grains composed of rock fragments).

Sandstones vary in color & coarseness. Common sandstone colors are brownish, grayish, reddish, orangish-brown, and whitish. Coarse-grained sandstones will feel the roughest when rubbed with a finger. Fine-grained sandstones are far less gritty feeling.

Some sandstones are hard, dense, and tight, which is the result of mineral cement (usually quartz) completely filling the spaces between grains. Many sandstones still have significant porosity between sand grains. Moderately- to poorly-cemented sandstones often have their porosity filled with groundwater, crude oil, natural gas, or a combination of these three fluids.

Sandstones have significant environmental and economic importance in human society. Many groundwater aquifers consist of sands or sandstones. Many petroleum fields have sandstones as the dominant reservoir rocks for oil & natural gas. Well-cemented sandstone is one of many types of building stone used around the world.