Rattleback / Celtic Stone

Rattlebacks can only freely spin in one direction and were used by celtic priests for decision making or manipulation.

https://youtu.be/8kRUkY_dQBo

This is an easy to print, symmetrical rattleback with super secret technology inside to achieve the desired spinning direction.
Prints without support, needs 100% infill.
The downside has to be smooth for the rattleback to work correcty, simply use some sandpaper.
Spinning direction is indicated with right (clockwise) and left.