The Crystal Temple's highlights is, not only the rooms of the Crystal Gems, but the carved giant woman structure on the outside. Many have guessed that the giant woman outside is the fusion of all the Crystal Gems (Rose, Ruby, Sapphire, Pearl and Amethyst). This is most likely true because all the gemstone placements of the giant woman is located on the gemstone placements of the Crystal Gems.
Deny it or not, we all want to see the giant woman in action. And we like to see it in the Rebellion, fighting small Homeworld women. Oh, wait! Stop right there! This massive drink of water didn't fight in the Rebellion. But why? Remember, Amethyst was still in the Kindergarten when the Rebellion was going on.
But when was it used? This is the question the theory will revolve around. So when was it really used?
During the episode "Historical Friction", Jamie and Steven presented a play focusing on the origin of Beach City. The play was originally written by Mayor Dewey but Pearl revised the play, since what Mayor Dewey had written was false (at least, some of it).
Let's focus on the play! It was told that a giant woman saved Bill Dewey's great(x7) grandfather, William Dewey, from a storm. Now, let's not believe the silhouette of the giant woman in the play, as it wasn't just true enough. It can't exactly look like that and plus, Jamie and Steven got Pearl's revision a day before they presented the play, so they didn't have enough time. Though we can depend on some facts like the fusion have four arms.
Who is this fusion? Fusions of the recent Crystal Gems couldn't be the fusion, but fusions of the former Crystal Gems could be. Rainbow Quartz wouldn't be, as the silhouette has four arms. A fusion between Rose and Amethyst would also not be an option because Amethyst was young (not of age but her state of mind). A fusion between Rose and Garnet would be, since Garnet is like the second strongest among the gems that time.
A fusion between Rose, Pearl and Garnet would most likely be an option; the three are very responsible.
Now, a fusion between all of the Crystal Gems' members is a suggestion all would make. Two or three gems can't possibly save someone from a storm. Also, fusions between Rose and another gem would maybe be small or a bit tall because everyone trusts her and she trusts them (remember, the stable the fusion is, the humanoid the fusion will look [based on my observations]).