If Gertrude Stein had been a chemist, she might have said, “Calcium carbonate is calcium carbonate is calcium carbonate.”
Sure, clams and oysters make their shells out of calcium carbonate. But chemically speaking, it doesn’t matter whether the calcium carbonate in the supplement bottle came from an oyster bed or a bed of limestone, which is also made of calcium carbonate.
Neither is more “natural” (whatever that means) than the other. Oysters incorporate a bit of non-mineral matter in their shells, however, so calcium carbonate from other sources might be a bit purer.
Calcium supplements are sold in other chemical forms besides calcium carbonate (read the labels). But weight for weight, these other forms contain less calcium than calcium carbonate does, and it’s the actual element calcium that you’re after; never mind the rest of the stuff.
Calcium carbonate contains 40 percent calcium by weight, while calcium citrate contains 21 percent, calcium lactate contains 13 percent, and calcium gluconate contains only 9 percent calcium.
Now you can figure out which supplement on the shelf gives you the most actual calcium for your money.