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 or if the technique was developed separately. In the 17th and early 18th centuries, the term "shagreen" began to be applied to leather made from sharkskin or the skin of a rayfish (probably the pearled ray, "Hypolophus sephen"). This form is also termed sharkskin or galuchat. Such skins are naturally covered with round, closely set, calcified papillae called placoid scales, whose size is chiefly dependent on the age and size of the animal. These scales are ground down to give a roughened surface of rounded pale protrusions, between which the dye (again, typically green vegetable dye) shows when the material