Cementation involves ions carried in groundwater chemically precipitating to form new crystalline material between sedimentary grains. The new pore-filling minerals forms "bridges" between original sediment grains, thereby binding them together. In this way, sand becomes sandstone, and gravel becomes conglomerate or breccia. Cementation occurs as part of the diagenesis or lithification of sediments. Cementation occurs primarily below the water table regardless of sedimentary grain sizes present. Large volumes of pore water must pass through sediment pores for new mineral cements to crystallize and so millions of years are generally required to complete the cementation process. Common mineral cements include calcite, quartz, and silica phases like cristobalite, iron oxides, and clay minerals; other mineral cements also occur.
Calcite cement in an ooid-rich limestone; Carmel Formation, Jurassic of Utah
Sandstone is a clastic sedimentary rock composed mainly of sand-sized silicate grains. Sandstones comprise about 20–25% of all sedimentary rocks.
Cut slab of sandstone
Alcove in the Navajo Sandstone
Kokh-type tombs cut into the multicoloured sandstone of Petra
Sand grains of quartz with hematite coating providing an orange colour