Presuming the piano is pre-1930's (the keys were topped with plastic starting around that time), and you have true ivory keys, they are called naturals, and they have only a thin layer of ivory on the top and front of the keys.
The effort to remove the ivory isn't worth it, since you wouldn't get much ivory, certainly not enough for a pen. It is also brittle, so expect damage.
Piano techs and some stores keep the key tops for repairs.
Keep in mind, all of the laws governing ivory. The laws on ivory are very strict, much more than, even the raptor laws.
There are plenty of alternatives to ivory, so for myself, the risk isn't worth the reward.
Also, I believe hide glue was used for the tops, originally.