Some of the statues in the Lunar Spire seem to depict fusion Gems - with extra pairs of arms. One of them makes me think of Opal, especially since she has so much simbolism with goddesses and mythology. And the Lunar Spire belongs/belonged to Homeworld, not to the Crystal Gems. 

This leads me to think about a really interesting plot - Opals being a very important and rare kind of Gem on Homeworld, and during a mission, Pearl and Amethyst will have to stay fused into Opal for an extented period of time, further improving both Opal's development as a character (she's really awesome and is totally needing and deserving some) and Pearl and Amethyst's relationship. This would also practically confirm the theory that some of the fusion Gems seen can exist as a single Gem with a single gemstone. I particullarly don't believe in this theory, at least right now, but it would certainly be interesting! And I only say this because we've seen that Homeworld Gems are disgusted by different types of Gems (this led me to think that maybe after so many years, they would have passed from that hatred, but Peridot still was surprised by seeing a Ruby fused with a Sapphire when she came to Earth), therefore, if Opals really were important and "sacred" on Homeworld, they would be single Gems, not fusions of Quartz soldiers (Amethysts) and Pearls.