This is a simple dice, printed on a corner to make a nice even surface on all faces, but with a spherical cavity in them.
If you put a spherical magnet in the cavity during printing you end up with some special dice - magnetic but not biased.
This actually works quite well with a ball bearing rather than a magnet, making dice that don't roll too far, and have a nice rattle.
Prints with no extra support on makerbot, the support ring on base snaps off.
Drop a small spherical magnet in at about 75% print. Suggest using a second magnet underneith to hold in place while finishing printing.