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Calcium Carbonate
Calcium carbonate makes up between 5 to 7 percent of the earth's crust. Compressed calcium carbonate is also known as chalk, limestone or marble, depending on its properties and consistencies. Calcium carbonate as commonly found in natural formations -- such as the infamous "white cliffs" of Dover -- actually contains millions of fossilized mollusks from ages past.
Pearls
As filter feeders, clams filter their oxygen and food from seawater through the mantle. Some undigested food over time gets covered by layers of nacre -- crystallized calcium carbonate, otherwise known as "mother of pearl" -- and bound together by conchiolin, an organic protein. Thousands of these concentric layers of nacre, building up over months, produce pearls.
Environmental Effects
Seashells primarily consist of carbonate ions, which frequently depend on environmental variables. Reduction of these ions, such as through increased acidification of the world's oceans, affects the shells of many sea creatures. According to a Woods Hole Oceanographic Institution study published in the November 2010 edition of "Oceanus Magazine," the shells of some organisms became thinner while others became harder, depending on how well they converted CO2 into shell-building material.