this large 85mm diameter spherical D15 is for the true Math geek. Numbered with all 15 primes of Monstrous M theory it is sure to get people talking. Even though the landing surfaces are all flat there is a large spherical weight which will help the die always land on a valid side.
Place a die of your chosing inside the die and screw it shut. You now have a 2 part die. Numbered 0 to 5 so if you place a D10 inside you can get a value from 0 to 59.
A D8 with the phases of the moon on it. It is made of all circles because we all know that even when the moon is a sliver it is still a sphere. The shape is completely fair.