Basalt is a volcanic stone – it forms from the lava that volcanoes spew out when they erupt. Basalt is a very hard, black stone. It makes a good stone for grinding wheat and barley and millet into flour, and by the Stone Age people who lived in places where there was basalt were already selling basalt grindstones to their neighbors, and even to people living pretty far away. Basalt is heavy, but people moved it around on boats, so they didn’t have to carry it themselves.
By the beginning of the Bronze Age, stone carvers in Ancient Egypt also used basalt to make statues. When you polish it, basalt makes very dramatic looking shiny black statues
In the Roman Empire, the Romans used basalt for roads, because it was very hard so it would last a long time. They still used basalt for grinding stones, too. Roman traders often shipped basalt (bu-SALT) grinding stones on sailing ships to markets all over the Mediterranean, so people could have good grinding stones even where there weren’t any volcanoes.
In the Roman Empire, people made more complicated basalt grindstones. This one, shaped like an hourglass set over a cone-shaped piece underneath, is a typical Roman grain mill.
Volcanoes, by Peter Francis and Clive Oppenheimer (second edition 2004).