Mammoth Ivory is from the tusks of the now extinct Mammoth. They are usually found in the Siberian tundra, Aleutian islands, and Alaska. Being from an extinct species it isn't as strictly regulated as Elephant ivory. However since that box is from Europe in the 1600's it most likely has Elephant ivory that would have been much easier to obtain at that time. Ivory has become a delicate subject as I recently discovered. Since your friend's snuff box is an heirloom that he probably doesn't want to sell it shouldn't be problem. However selling Ivory objects on the open market is now pretty much prohibited around the world in an effort to squash the Ivory trade.