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.