The shells of oysters and many other sea creatures, like lobsters, have shells made up of calcium carbonate. Therefore, if you eat a lot of seafood and are throwing out the shells, you are wasting valuable garden fertilizers. I brought home a bag of oyster shells last night to add to my compost pile. What I am going to do is to first wash them out so that the local critters don't take an interest. Then I'll sandwich them between a couple old rags and smash the shells into little bits. I smash them because a whole shell is a lot to break down in a compost and it would take forever. Next I can just throw them right into the compost pile. You can do this with many kinds of sea shells and things like eggs as well. They will all add calcium carbonate to your soil. Calcium carbonate is basically what lime is just in case you didn't know that already.