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
Ostrich leather and style are vague and often simply a subjective opinion of the tanner or buyer. A defect can be such things as a hole, scratch, loose scab, a healed wound or bacterial damage. The World Ostrich Association has a document with full definitions of defects for each grade. The World Ostrich Association also have a document entitled "Factors Influencing Skin Quality" that can be accessed from the Association's industry standards menu. Traditionally, fashion has driven the demand for ostrich leather. Fashion houses successfully used ostrich leather in handbags for many years. Most designer brands have at least one purse made