Roman rotary grain mills in the Mediterranean area are mainly of two styles: hourglass-shaped Pompeian-style mills and flatter, cylindrical millstones. Many of these millstones are made of igneous rocks imported to the sites where the mills are found, and therefore they may be important indicators of resource procurement and trade if the source of the rock of manufacture can be traced. Potential source rocks and millstones were characterized for three types of igneous rock:1) reddish rhyolitic ignimbrite; 2) leucite-phyric lavas and 3) generally fresh gray vesicular lavas of basic-intermediate composition. Petrographic examination in thin section and chemical analysis were performed and major and 15 trace elements were determined by low-dilution fusion wavelength-dispersive x-ray fluorescence analysis. 146 millstones and lavas from archaeological contexts were studied, and 113 of these were successfully assigned to geological sources from some 16 geographic areas throughout the Mediterranean region. -- AATA