Long before there were fidget spinners, there were "worry stones." These examples of the human need for tactility were smooth rocks with a natural indentation on one side; the idea was that you rubbed your thumb in the indentation as a way to soothe stress out of your brain.
Good luck on finding their origin—depending on who you listen to, they were carried by the ancient Greeks, or the Tibetans, or the Irish, or the Native Americans. Today machinists and specialty mints make "worry coins," metal discs that are not currency but that have a machined indentation for thumb-rubbing on one side.
Look at these beautiful examples from California-based knifemaker Meton:
Meton makes them in a variety of colors and finishes:
Depending on the metal and finish you select, these go for anywhere from $30 to north of $200. You can see an exhaustive lineup of their offerings here.
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