Oysters ultimately rely on the carbon present in water as raw materials from which they create their shells. The way these shells are generally made is that these organisms pick up calcium, they package it in proteins and then then direct these proteins to their exterior. Once there, these proteins release calcium in the presence of carbonate ions, and then slowly secrete these building blocks, gradually forming a shell that is made up of 90%+ calcium carbonate, some organic material, and trace amounts of other elements such as iron and manganese.