Summary

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.

Print Settings

Printer Brand:

MakerGear

                                            <p class="detail-setting printer"><strong>Printer: </strong>                     <div><p>M2</p></div></p>                                                                                <p class="detail-setting rafts"><strong>Rafts: </strong>                     <div><p>No</p></div></p>                                                                                <p class="detail-setting supports"><strong>Supports: </strong>                     <div><p>No</p></div></p>                                                                                <p class="detail-setting infill"><strong>Infill: </strong>                     <div><p>100%</p></div></p>                                                        <br>                        <p class="detail-setting notes"><strong>Notes: </strong>                    </p><div><p>The Infill has to be 100% for the rattleback to work really well.<br>Resolution is unimportant since you have to use sandpaper anyway.</p></div>

Rattleback / Celtic Stone