Sure, I'll back it up - it's called common sense. Schala gave Magus a pendant, correct? This pendant is clearly different than Schala's pendant that Marle is in possession of, since this pendant survived the original Ocean Palace incident and was passed down the Guardia royal line, correct? This pendant is also clearly the same one as the one that Schala was sent to the DBT with in the new Ocean Palace incident. Follow so far? When Schala duplicated herself and created Kid, presumably the pendant around Kid's neck in Guardia forest is the same as Schala's pendant from the DBT - right?
And we already know from the script of Cross that the Astral Amulet is a completely different item.
So, conclusion: Kid's pendant=Schala's pendant=Marle pendant which does not equal the pendant that Magus has which does not equal the Astral Amulet. They are three different items.