The age of the Crystal Temple and it's implications

One basic thing to start with. The Crystal Temple is in the image of Obsidian, meaning it was constructed at least a few hundred years after the end of the war. Obsidian includes Amethyst, and she hadn't hatched until then. Because Obsidian (the statue) and Obsidian (the finale fusion) are near identical, and fusions are down to be very dependent on their participants, we can assume that no alternative gem combination could have assumed the exact same form even if they had another Amethyst

This creates a problem- who built it? None of our gems are particularly skilled builders and the only helpful abilities among the individuals and known fusions is strength. Therefore I posit that the temple was built by one or more previously unseen fusions with more relevant abilities. I have dubbed the candidates by gems elementally similar to their components

* Tourmaline, Ruby/Amethyst, abilities: heat, shape shifting, consumption. Able to eat large quantities of rock, covert it to lava, and possibly act as a mold † * Turquoise, Sapphire/Pearl, abilities: future sight and precision. Able to work with a material as temperamental as stone without unwanted breaks

Rose would either supervise or fuse with Tourmaline to add size. And this would definitely be a Rose driven project. Ruby and Pearl would care more about representing their paramours while Sapphire and Amethyst wouldn't see the point. Assuming there is no similar structure miles away showing the fusion of Rose, Pearl, Sapphire, and Ruby, the choice to model their home after Obsidian represents giving up on finding any other gems. By this we can probably assume they waited for hundreds of years after finding Amethyst to begin construction so I would estimate the temples are to be less than 4000 years though probably much younger.

The oldest depiction we have of the temple is from Buddy Budwick. His drawings show the temple to be a bit of a wreck by the 19th century and in almost identical condition to modern day. We thus know the statue was old 200 years ago. The fact that there damages have not been repaired implies that repair was either impossible or fruitless. There is no evidence to suggest new parts were ever made based on the lack of extra hands lying around. I see no evidence to guess why the statue was left to collapse other than a growing sense of hopelessness among the Crystal Gems.

All that said, the interior properties of the temple (it's ability to add rooms as seen with the training course) would seem to mean it was intended to be able to house more gems. There's no way to tell when the interior was created but I believe it does not predate the war. Most of the gem structures have been left abandoned except where necessary to prevent danger or total failure (temple of the moon goddess). Crystal Gem strongholds that predate the diamond attack may hold to many memories but were also likely infested with corrupted gems for quite some time before a reliable process for containment was developed.

On the other hand, the temple would be the perfect candidate for a wartime base and no alternative base was ever shown outside of Bismuth's forge. Since Bismuth appeared to operate alone and the Heaven Beatle lived in her own tiny house on a mountain, it's possible than most of the old Crystal Gems chose to homestead independently across the planet, making the temple as possibly a fallback point.

Weirdly I feel that the construction of a crystal based semi extra dimensional home space to be easier than a stone statue as only one plays by any rules. So I have no convincing argument for why the inside of the temple would require any more explanation than "a magic rock"

† the temple had no apparent sign of brickwork and it's breaks are jagged so it was likely cast. The formation of the hill it sits against rules out there having been a large enough rock formation to carve the statue