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 is coloured from the other side. This latter form of shagreen was first popularised in Europe by (d. 1774), a master leatherworker in the court of Louis XV of France. It quickly became a fashion amongst the French aristocracy, and migrated throughout Europe by the mid-18th century. "The appeal of shagreen in Europe was closely bound up with the way the translucent skin absorbs colour, but in Japan, its whiteness was the measure of value". Scholars concur that its popularity in Central Asia, where green footwear made of shagreen was popular into the 19th century, probably came as the result