Short answers, pulled from the story.
Limestone is a carbonate sedimentary rock composed mostly of the minerals calcite and aragonite, which are different crystal forms of calcium carbonate. Most limestone is otherwise chemically pure, with clastic sediments of fine quartz and clay making up less than 5% to 10% and organic matter typically around 0.2%.
Limestone forms when calcite or aragonite precipitate out of water containing dissolved calcium, through both biological and nonbiological processes. For the last 540 million years biological processes such as the accumulation of corals and shells have likely dominated, mostly in shallow marine environments like continental shelves and platforms.
Limestone is slightly soluble in rainwater, so exposed limestone erodes into karst landscapes featuring pavements, pot holes, cenotes, caves, and gorges. Water and organic acid slowly widen cracks over thousands or millions of years, dissolving the calcium carbonate, and most cave systems run through limestone bedrock.
Limestone is used in construction, as the raw material for lime and cement, as road aggregate, as a white pigment and filler in products like toothpaste and paint, as a soil conditioner, and as a reagent in flue-gas desulfurization. Many porous limestone formations also serve as petroleum reservoirs, holding roughly a third of all petroleum reserves worldwide.
Limestone is classified mainly by two schemes. Robert L. Folk built a system around the detailed composition of grains, matrix, and cement, while Robert J. Dunham published a system in 1962 focused on depositional fabric and original porosity. A revised classification was proposed by Wright in 1992.
Limestone is relatively soft, with a Mohs hardness of 2 to 4, yet dense limestone can have a crushing strength of up to 180 megapascals compared with about 40 megapascals for concrete. Its density ranges from 1.5 to 2.7 grams per cubic centimeter depending on porosity, which varies from 0.1% in the densest limestone to 40% in chalk.