Apparently Rose is actually Pink Diamond. They'd better have a darn good explanation, because right now she's looking like even more of a horrible character. However, narrative things aside, here are some factual things that don't add up:

-"You're some kind of Quartz, right?"

Peridot was able to identify Steven's gem as a Quartz.

Not to mention, all the Rose Quartz gems look like that. Pink must have designed them after her own gemstone, which is just kind of a lame explanation imo. But hey, what can ya do.

Rainbow Quartz

With this, any semblance of fusion gems having a predictable outcome based on their components goes out the window. Rose wasn't even a Quartz.

By this logic, an Amethyst and a Jasper could make an ruby or something.

Shapeshifting

This comes out of nowhere. Apparently the normal rules of Shapeshifting don't apply to Pink Diamond, which is a really contrived way to give a twist shock value (It does make you think about Greg's "you know how I feel about Shapeshifting" line though...). It comes out of nowhere and is really unsatisfying.

Don't get me wrong, I love these episodes, and I bet they'll do some great things with this concept in the future... However, the explanation for how this is possible makes no sense and goes against established rules.