Growing evidence suggests that Calcium Carbonate, which is chalk or limestone, is not a biologically appropriate form of calcium for human metabolism.  The primary justification for ingesting Calcium Carbonate is to "support bones," however, Lancet and the British Medical Journal, recently published the results of two extensive clinical trails which concluded that Calcium plus Vitamin D does nothing to prevent bone loss.  Calcium as found in chelated form, e.g. calcium citrate, calcium bisglycinate, calcium asporatate, or in its natural state as Food, is much more readily absorbed and utilized within the body, and does not have the risk factors associated with inorganic calcium ingestion, i.e. calcification of soft tissue, osteoarthritis, constipation, kidney stones, hypertension and various other side effects of poorly utilized calcium.  

Additional Reading: The Dangers of Calcium Supplementation