TAPERED WIDTH 2.25" to 1.50"
THREADED HOLE WIDTH .75"
HOLE DEPTH 1.75"
And even if it was wood, there should probably still be a brass or copper fitting. That's why he should go back in with a metal detector. Any mystery is a good mystery.It does not have the end wear expected on a pestle.
The thread does not look like a wood thread. Most brass tube is threaded with 26 tpi. What is the pitch of the thread ?
If marble, the hole would be drilled and tapped, then a bolt in a lathe would hold the article while the profile was cut and polished.