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 a uniform surface), sometimes with inlay work of different coloured shagreen. Shagreen was a very common cover for 19th-century reading glasses containers as well as other utensil boxes from China. The early horse-skin variety of shagreen was traditionally prepared by embedding plant seeds (often "Chenopodium") in the untreated skin while soft, covering the skin with a cloth, and trampling them into the skin. When the skin was dry, the seeds were shaken off, leaving the surface of the leather covered with small indentations. Sources are not clear whether this was being done to imitate pearl ray-skin shagreen from East Asia