Calcium carbonate, or CaCO3, comprises more than 4% of the earth’s crust and is found throughout the world.  Its most common natural forms are chalk, limestone, and marble, produced by the sedimentation of the shells of small fossilized snails, shellfish, and coral over millions of years.  Although all three forms are identical in chemical terms, they differ in many other respects, including purity, whiteness, thickness and homogeneity.  Calcium carbonate is one of the most useful and versatile materials known to man.