There seems to be more to Rose Quartz, beloved leader of the Crystal Gems, than meets the eye. The series has dropped some pretty heavy hints that she used to be a very important Gem: Pink Diamond. We can’t be sure at this point whether Pink Diamond is her gem type and Rose Quartz is just an assumed name, if Diamond is just a title leaders take, or if leaders go by their own names and Yellow Diamond simply happens to be a diamond.
The origin of the title
Even quite early in the series it was common for fans to theorize that there was a Diamond or group of Diamonds at the top of the Gem hierarchy. The diamond is probably the most well-known gem, and is certainly the hardest, so it was easy to assume that something important would be done with the gem.
Ronaldo’s epiphany about the Diamond Authority lent new weight to the theories. He had been right about the polymorphic sentient rocks (if not their exact plan, it would later turn out), so fans started using the term as the label for the Gem rulers.
The Return and Jailbreak hit us with two pieces of information. First we learned that Jasper reported to Yellow Diamond. Then the floor of the room that Garnet and Jasper fought in contained a symbol that included three triangles: one each blue, white, and yellow slightly overlapping to make a small green triangle at the center. It was this that made certain that the label “Diamond Authority” stuck among fans. (This symbol also exists, without color, in the pyramid temple in Serious Steven.)
At this point fans observed that Rose Quartz used a pink triangle as her personal symbol; it appeared on the Crystal Gem battle flag from Rose’s Scabbard, Rose’s sword and shield, and the control panel in her secret armory. This was enough for us to assume that Rose Quartz was Pink Diamond, but the series has since thrown us even more to chew on and at the time we hadn’t noticed that the rhombus on Pearl’s space suit in Space Race was colored pink, just as Jasper’s is yellow.
The new evidence
Of course Sworn to the Sword gave us a wonderful bit of information: the “Diamond Authority” (like we said, the name stuck) used to have a fourth member before the Crystal Gem rebellion. Blue, White, and Yellow still existed, and there was a Pink as well.
Obviously this is very easily interpreted as evidence for Rose Quartz being Pink Diamond. The sky arena where the insignia is found was made before the rebellion and shows there being four rulers; the pyramid temple was made after the hostilities had started (it depicts Rose Quartz facing a Gem with a triangle for a personal symbol in single combat) and shows there being only three. Rose Quartz is the only pink Gem we know of so far, and considering how important she is to the backstory of the series it would be amazingly surprising to find out that another Gem was Pink Diamond.
There is a small unanswered question, however: with four members the Diamond Authority seemed to be represented with squares rather than triangles. Were their personal symbols always triangles, or did they change from squares to triangles when Rose Quartz betrayed them and Rose changed her symbol as well in an effort to claim legitimacy?
That is, like we said, just a small question. The evidence speaks plainly and quite loudly: Rose Quartz was clearly “Pink Diamond”, meaning she was one of the four leaders at the top of Gem society. It may be the true name of her gem, a title she took just as one might take up the title of Queen, or leaders may not actually have to be called “Diamond”, but she was clearly one of those four leaders.