I believe this was made as a spirit cup and dates to the Civil War era, based on a few others I have turned up that are similar. And I believe it is French flags that are cross the anchor at center, with finials evoking the fleur-de-lys--representing the French navy. Likely made by a sailor aboard a ship, and, despite a rim chip on one side, quite a lovely thing, which would make a great dice cup now.   

2 1/2" t x 1 3/4." Chip to the rim as documented and what looks to me like a very old repair to the bottom of the cup--the flat of it sits at a bit of an angle, though is well attached. The bone itself shows some discoloration here and there as documented.