This study concerns 47 hourglass and 25 rotary-hand millstones from Pompei. All of them are made of volcanic rocks. Most samples were chemically analysed and all were studied by optical microscopy.
Out of the rotary-hand millstones, 20 are composed of leucite basaltic trachyandesites belonging to the oldest eruptions of Somma-Vesuvius, 4 are mugearites and hawaiites from Etna, and 1 (of uncertain provenance) is a leucitic tephriphonolite. Among the hourglass millstone samples, 29 are leucitic phonolites from the Orvieto quarries discovered by Peacock (1986), and 18 are leucite basaltic trachyandesites identical to the rocks making up the rotary-hand millstones.
The typology of the hourglass millstones varies between the different manufacturing areas. According to the morphological classification we propose here Orvieto millstones belong to the A type and Somma-Vesuvius millstones to the B type.
Therefore, our research demonstrates that millstone trade was frequent and important in Roman times.