He's a Riddle For You To Figure Out

This is an old Italian riddle from 1300-1900 AD.

A useful thing, hard, firm, and white,
Outside in shaggy robe bedight;
Hallowed within right cleverly,
It goes to work both white and dry.
When after labour it comes back,
You'll find it moist and very black;
For service it is ready ever,
And fails the hand that guides it never.