Saphire is definitely Garnet's vestigial form, prior to fusing with Ruby.

The facial structure and the lips are the same as Garnet's. Also, the dress Sapphire is wearing is similar to the silhouette shown when Alexandrite broke apart. One second frame showed four different Gems and four silhouettes, before being reconstituted into Garnet, Pearl and Amethyst respectively.

We know Garnet can open the red and blue lights of the temple. Similarly two of her three eyes match the doors on the temple. This proves that Garnet is indeed a fusion.

You may argue, "Garnet doesn't have four arms!"

So what? Stevonnie didn't have four arms either and it was a fusion as well. Besides if we really wanted to get technical, all Gem fusions should have four eyes or at least display heterochromia like Garnet.

The characteristics a Gem may show upon fusion are inconsistent and undefined by a set pattern, so we can't discard the Garnet fusion theory solely based on that assumption.

Now, we see Sapphire's hair covers her forehead, maybe she's a cyclops! -Also, they're lesbians holy I love this show-

plus peridot exists