I would say stone because the categories are based on behavior, not composition. Bauxite is stone. Aluminum (if you could extract it in the middle ages) is metal. Sand is the same as dirt, but glass is with stone. Hemetite is stone, iron pyrite (fools gold) is metal, but rust is also metal. Silicone would presumedly be metal as well, even though it is also stone (quartz, glass) and dirt (sand). As to precious stones, that gets into medieval cosmology and the fact they purportedly rproduce far beneath the earth, and categorically semi-alive...