These great magnets are coated with a durable layer of protective rubber coating. The 3/4" x 3/8" dimensions are the finished dimensions of the rubber coating. These discs can be slammed together many times without damage. They won't mark or scratch surfaces they come in contact with.
The rubber coating is very water resistant, but not recommended for underwater use, as the rubber seal is not guaranteed waterproof. The rubber also helps to limit slipping on smooth surfaces as we describe in our Leverage & Friction