BIRKENSTOCK. 1013967. YAO BIRKIBUC. MOCHA. An asymmetric eye-catcher The BIRKENSTOCK Yao is popular with people who like unusual fashion and is sure to turn heads. The upper is made from the skin-friendly hard-wearing synthetic material Birko-Flor in a sophisticated nubuck look which boasts a texture and color that could almost be mistaken for real leather.