Here's an idea for you, altho it's outside the scope of this forum:
make them out of stone.
At least 2 types of stone I know of can be turned on a lathe -
soapstone and alabaster. If you ask a rock hound, they will be able to
tell you the hardness of many other available stones similar to
soapstone.
If you search the web, you will find sites of those who turn stone,
contact them for help.

I really think you're barking up the wrong "tree", pun intended, by
using wood for this aplication.

Another possibility: turn from any wood, make a mold, and use poured
concrete for the final product.