In the geological sense, millstones are siliceous accidents in a sedimentary basin that were used for the manufacture of stone millstones. Their extraction, initiated in the 15th century, became widespread in the 18th century. It concerned limestone and especially sandstone rocks whose porosity facilitated quarrying and grinding. A family of choice for millstones, the sandstone rocks used include sandstone with limestone cement, sandstone with siliceous cement, or even metamorphosed sandstone. Their percentage of silica increases their resistance, and they must be insensitive to dissolution and humidity.