Fish scale the surface, interacting only with the riblet tips, not causing any high-veloctiy flow in the valleys. Since this high velocity flow now only interacts with the riblet-tip, which is a very small surface area, the momentum transfer which causes drag is now much lower than before, thereby effectively reducing drag. Also, this reduces the cross-stream velocity fluctuations, which aids in momentum transfer too. The rough, sandpaper-like texture of shark and ray skin, coupled with its toughness, has led it to be valued as a source of rawhide leather, called shagreen. One of the many historical applications of shark shagreen was
Shagreen of its introduction from China perhaps during the 16th century rule of Suleiman the Magnificent. Three items made of shagreen can be found in the Topkapi collection. Since the 18th century Shagreen has typically been used for covering portable items such as luggage, toilet cases and other boxes, the hard and robust looking finish seeming to assure its reputation for standing up to rough handling; book coverings are also known as well as larger pieces of furniture. The water resistant qualities ascribed to the finish are probably why there are many toiletry cases and associated objects such as shaving kits,