These worry stones (sometimes called relaxation stones or palm stones) were developed by Jeff Parish, a mechanical engineer, 3D printing enthusiast, and designer responsible for many of the design features of our ReliaBuild 3D printers. Worry stones are used by holding the stone in your hand and rubbing the indentation with your thumb to calm nerves and relieve stress. They can also be used as a meditation focus. He designed the stones for his wife and printed them on his ReliaBuild 3D printers.
Jeff designed the stones in trueSpace 6.6, then used thermally calibrated PLA filament at 0.05mm layer height to produce a super smooth surface. He finished the stones with 800 grit sandpaper to give them an ultra-smooth finish.
Full specs and printer settings available at https://www.thingiverse.com/thing:2702654.