Tumblestones or "tumbled stones" as they are also called are made by "tumbling" rough crystals in grit for a number of days in a giant tumbler machine. As time passes, finer and finer grit is used until the edges are smoothed and the crystals start to become polished. In the end they end up beautifully smooth. Due to the tumbling process, each stone will be unique in shape, size and colour.
