Another fine artifact shown from the “A Day in Pompeii” travelling exhibit at the Denver Museum of Nature and Science:
This millstone, made of lava rock (with a modern recreation wooden handle) would have been about the right size for a household or small shop. Its owner poured grain into the hole in the top, and then used the handle to rotate the upper stone. The grain worked its way into the gap between the stones where it was crushed.
From the House of Fabius Rufus in Pompeii.