This set ("Wild Magic") was an interesting one. The dice were double-cast - that is, we started with blank dice, applied a decoration, then cast them in regular moulds to seal the decoration in.
For decoration, the blanks were hydro-dipped. This is a process where drops of a special ink are placed onto the water surface. The blank die is then lowered carefully into the water and the ink transfers onto it.
You can end up with some really cool effects with this method!