Calcium from oyster shells is Calcium Carbonate and is quite hard on your tummy. Calcium that is chelated is better apsorbed and easier on your system. Most of us get our calcium and magnesium together in one supplement - the two work in concert. It's the magnesium that lets the calcium do it's job. RDA for magnesium is 400-500mg a day depending on age; for Calcium it is 1000-1500 also depending on age and whether you are nursing/pregnant.

Nat