Hollow dice in a creative design with counter weights inside to make it a fair dice.
Why counter weights? Because on each side there is a different number of holes, each side has a different weight.
So without counter weights the center of mass would not be in the middle and therefore the dice would not be fair.
The calculated counter weights inside ensure that the center of mass is in the middle and therefore each side has the same probability to show up after the dice is thrown.
Also available in size S (with photos) and size M.