The old millstone was the instrument traditionally used for pressing or grinding (°) the olives, which then became olive paste.
The pressing or grinding was done by crushing the olives on a rimmed base with a big granite wheel pulled in a circular movement by a beam drawn by an animal.
You can still find old millstones in Tignale, reflecting their widespread use over the years. Many are now used as garden ornaments.

(°) Contrary to what you might think, the juice is not released by the crushing, but from the rubbing of the cutting edges of the fragments of the olive stones on the pulp of the olives.