Limestone

Limestone was a sedimentary rock formed about 500 million years prior to the 20th century, during the Carboniferous Period on Earth. By the modern era, it became a valuable enough resource that companies like the Stockbridge Limestone Company existed to extract it. (COMIC: The Stockbridge Horror) Limestone was mixed with clay to make cement. (TV: Destiny of the Daleks) The Fourth Doctor once defined blackstone dolomite as "Jurassic limestone." (TV: The Hand of Fear)

Limestone was also known as calcium carbonate. (AUDIO: Eye of the Storm)