Yes, there is a difference. Our oyster shell is shipped from Chesapeake Bay so the crushed product contains the minerals that oysters absorb from there. Limestone flour, the product used mainly in the UK, is mainly calcium carbonate but varies from one location to another and may contain some dolomite and other minerals. But basically your hens need calcium carbonate and will get this from either product. I remember my uncle used to go to the shoreline and gather all kinds of seashells and have his sons pound them down and put them in sacks for the hens. It's a lot of work, but as effective. Many layer feeds contain calcium carbonate, but I offer extra because the needs of some hens, particularly those that lay big eggs, vary.