There is speculation if there are more than one of the same kind of gem. On top of that, it is hard to say that it will be a fusion of those two. Just like Pearl and Amethyst form Opal, it is hard to say that it won't be something like that.
To be honest, I hope that Sardonyx would be a fusion of Pearl and Garnet since it hasn't been shown yet or maybe one of the gems and Steven finally fusing.