Originally posted by Banzai Seppuku:

Several people commented that piggys follow the parents. The 2 star and 1 star bred and made a no star. Now it was forced away from the stone and maybe that did something. Perhaps the magic is being close enough to the stone to make a X star. If they're too far they just make a no star. That would mean the rune stone is essential for star piggys. Be interesting to see if no stars can breed stars around a stone.

Rune stone just work for 1 time spawn for 2* 1* adult pigs , after that have nothing to do with breed
I have a breed tower with 4 2*boars in the top , always drops 2* piggies