Antler was used for buttons in the old days, but not cut off square that so common today.
Drill two holes crosswise near the tip. Then cut off the tip and sew it on, cutting the button hole cross ways to how you mount the tip.
Better then just a tip, shape both sides to form a proper toggle.
Another way is to cut a slab off one side, shape to a circle or square and sew that on as your button, finishing off flat or doomed on one side. Try and stay in the hard bone like area and not the core. Metal,horn or bone would have been more typical for the time frame.
If you want a cross cut button just hack saw a piece off and sand with fine sand paper, it will be strongest it there is little or no core. Most wont break even with a little core unless exposed to more pressure then you will likely get in a small bag.