A grindstone is a round sharpening stone used for grinding or sharpening ferrous tools. They are usually made from sandstone.Grindstone machines usually have pedals in which to speed and slow the stone to sharpen metal to the point of perfection.A rotary grindstone − the earliest representation thereof, which is operated by a crank handle, is shown in the Carolingian manuscript Utrecht Psalter; the pen drawing from about 830 goes back to a late antique original. The Luttrell Psalter, dating to around 1340, describes a grindstone which was rotated by two cranks, one at each end of its axle. Around 1480, the early medieval rotary grindstone was improved with a treadle and crank mechanism.