In addition to being very thick -- as much as four inches in some species -- shark skin is made up of tiny rectangular scales topped with even smaller spines or bristles, making the skin rough to the touch.
Shark skin was used in the past as an abrasive, for polishing wood. In Asia, it was used to decorate sword hilts. In the South Pacific, natives used it for the membranes on drums. Even today, because shark skin is so tough and pliable, it is used to make fine leather goods, including purses, shoes, boots and wallets.
Shark skin is covered with tiny scales, known as placoid scales. These scales resemble small shark teeth in both appearance and structure: there is an outer layer of enamel, dentine, and a central pulp cavity. (Biologists call them "dermal denticles," which literally translates into "tiny skin teeth.")
Sharks essentially have a built-in suit of chain mail armor that doesn't make them too stiff to move. The scales move and flex as the shark swims.
The shark skin's dentine layer is made of a hard, crystalline material, which is embedded in a soft protein. This is important because embedding a hard material inside a softer one combines the best properties of both: a material that is rigid without being brittle.
The structure of shark skin has another function besides protection. The streamlined shape of the scales decreases the friction of the water flowing along the shark's body by channeling it through grooves. The grooves are so closely spaced, they prevent eddies from coming into contact with the surface of the shark's moving body. This reduces the amount of "drag" as the shark swims, enabling the creature to glide farther on a given amount of energy. Scientists have found that the ridges created by shark scales can reduce drag in the water by as much as 8 percent. Golf balls and many military aircraft and vessels employ similar drag-reducing principles.