The mass of the Earth's crust is 59 percent silica, the main constituent of more than 95 percent of the known rocks. Other varieties include coesite, keatite, and lechatelierite. Silica sand is used in buildings and roads in the form of portland cement, concrete, and mortar, as well as sandstone.
Most limestones have a granular texture.
In many cases, the grains are microscopic fragments of fossil animal shells. Limestone has long fascinated earth scientists because of its rich fossil content. Much knowledge of the Earth's chronology and development has been derived from the study of fossils embedded in limestone and other carbonate rocks. Limestone also has considerable commercial importance. Limestones enriched in phosphate by the chemical action of ocean waters constitute a principal source of raw materials for the fertilizer industry.