Under normal circumstances, yes, a gem can only accept replacements of a similar makeup to their own, if they lose something permanently. Phos, on the other hand, seems to be able to accept any material... if they lost more of themself, they could probably use whatever to replace it.

[ Franken Phos gives them quite a bit of stress, all things considered. They look a bit exasperated even explaining it. ]

As you can probably tell, they have a bad habit of losing parts of themself. Believe me when I say there's no other gem like that.