The Bronnikov family from Vjatka, Russia is responsible for this hand-built and hand-carved mechanical watch made entirely of bone (except for mainspring, balance spring and pivots), which sold at auction in 2005 for $25,000. (Click image for detail.)
Also the makers of wood and ivory versions, the Bronnikov's earliest model appeared in 1837 and is rumored to have been purchased by the future Czar, Alexander II. Producing only one watch per month with approximately 500 ever made, of those only about 250 have survived today. The horological dynasty continued with Bronnikov sons Mikhail and Nicolai, who carried out the tradition through the 1800s into the early 20th century. More about the history and more photos here.