Sandstone is the 2nd-most common siliciclastic sedimentary rock on Earth. They have both environmental and economic importance in human society.
Sandstones are composed of sand-sized grains (between 2 mm and 0.0625 mm in size). The vast majority of sandstones are quartzose sandstones (dominated by quartz sand), but feldspathic sandstones (dominated by K-feldspar sand) and lithic sandstones (dominated by sand grains composed of rock fragments) are not uncommon.
Sandstones vary in color & coarseness. Common sandstone colors are brownish, grayish, reddish, orangish, 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. This is the result of mineral cement (usually quartz) completely filling up the spaces between sand grains. Many sandstones have not had cement completely filling the empty spaces between sand grains. So, the degree of porosity in sandstones varies. Moderately- to poorly-cemented sandstones often have their porosity filled with water, oil, or natural gas.
Many sources of groundwater (aquifers) in the world are sand or sandstone deposits. Many economic petroleum fields have sandstones as the reservoir rocks for the oil & natural gas. Well-cemented sandstones have traditionally been used as building stones in various parts of the world.
Sandstone (quartzose sandstone)
Graywacke (2.5 cm across) - this sandstone is a graywacke, which generally means a "dirty sandstone". This refers to the rock having a significant, fine-grained, muddy component mixed in with the sand grains. “Graywacke” is probably the most misused & misunderstood term in all of geology. The word often appears in older geologic literature, and modern geologists have been encouraged to avoid using the term.