The middle part includes an inner metallic middle part (10) and an outer middle part (20) made of a natural or synthetic mineral material, such as ceramics. The inner middle part (10) is itself made up of a top ring (12) and a bottom ring (14), each ring including a radial, peripheral extension (13, 15), whose opposite shoulders can mechanically lock the outer middle part (20), for example by screwing the two rings (12, 14) one inside the other.