Testing 10k rose gold. This seems like a shark tooth, probably from a sand tiger shark, which the Victorian sometimes used as talismans in jewelry. A hole was drilled from the front to the back and pierced with a gold pin which terminates in a flat head. The tooth moves (swivels) on this pin, but not much, and it's meant to do this, apparently. You can change the orientation of the tooth. Tooth is 1 1/8" long, total pin length is 3". COOL. c. 1880.