125 whole and fragmented millstones associated with 309 small millstone flakes (chipped off during use) were unearthed in the 2013 and 2014 excavations of a hydraulic ore mill at Brandes-en-Oisans (Huez, Isère, France). Ore grinding, a step taking place between crushing and washing, is part of the operational sequence of polymetallic metals (lead, copper, silver) production. Most of the millstones date to the first half of the 13th century. Others, reused in constructions, are slightly older (before 1210). Their typology is homogeneous with diameters between 80 and 130 cm. These millstones, often asymmetric, are characterised by dressing furrows and striations and show a high degree of wear and fragmentation. They are examples of an industrial application of hydraulic power and fall into the typological guidelines established by the first studies of medieval millstones.