Assuming that one very popular theory turns out to be canon. This might not be all that implausible.

To begin with, diamonds and quartzes are two completely different kinds of minerals in real life. Diamonds and Quartzes are also two completely different “species” of Gems in the show as well, as Diamonds are much larger than all other Gems and normally tend to have rhombus-shaped gemstones. Besides, even if we, the viewers, don’t see them all, there are still Diamond Authority murals everywhere and the characters still can see them: if Rose was depicted with other Diamonds on the Moon, Steven would have noticed and said something. One could argue that Pink Diamond took on a smaller form to disguise herself as a Quartz (why would she even do that?), but her fusion with Pearl still gives another Quartz, so she must be a genuine Quartz. In addition, Ian had confirmed on twitter that Rose is definitely a Quartz, essentially debunking the theory.