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
Chamois leather Chamois leather Chamois leather () is a type of porous leather, traditionally the skin of the chamois ("Rupicapra rupicapra"), a type of European mountain goat but today it is made almost exclusively from the flesh split of a sheepskin. The leather is favored for its gentle, non-abrasive composition and absorption properties. It has a range of uses: Imitation chamois leather is made using other leathers (such as the domestic goat, or pig), and synthetic chamois leather is usually made from polyvinyl alcohol or non-woven viscose rayon. The British Standard BS 6715: 1991 defines chamois leather as: "Leather made from the