Coasters with math imprints. Three examples are provided in the .scad file, but you can create any 2D openscad object and use it as an imprint.As usual, you can contribute on github too: https://github.com/tom111/PiIstGenau3D/tree/master/Coaster
Technical
Be careful to print them in ABS or PET, but not PLA, if you plan to put hot beverages on the coasters. PLA could melt.