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