As you can see from these projects, pretty much any kind of seashell works for crafting. The type of shell really depends on the project. For projects where you'll be gluing shells to another surface, pick ones that have a large, smooth surface area for best adhesion. Cowrie shells, scallop shells and cockle shells are good for making jewelry. I really enjoy crafting with oyster shells. They're good for making candles, oyster shell mirrors and ring dishes. Whatever type of shell you're working with, make sure they are clean and dry before crafting with them.