It's magnetite. Magnetite is a naturally occurring iron bearing mineral. It is extremely abundant in the ocean. There have been plans to harvest magnetite and other metal bearing minerals from the ocean, but the process is not very efficient and environmentalists always step in to block such harvesting becuase 'muh environment and sheeit. Carbon is not ferromagnetic so no form of carbon, including graphene, would stick to the magnet like that. Carbon does have some paramagnetic properties, but it isn't attracted to a fixed field magnet just like aluminum isn't either despite being paramagnetic as well.