So why are the Homeworld gems coming back to Earth? Restart the Kindergarten(s)? Possibly. I’d imagine that the Homeworld has a vast array of planets across the galaxy, and for all we know, outside of it (you don’t need big generational ships when your race is immortal). All of these planets could be hosting kindergartens. So why come to Earth?
Thanks to active geological activity, Earth has a lot of minerals. A LOT. Naturally, this would make for a lot of Gem formation potential. I’m sure they have other planets like Earth, but my guess is that they’ve depleted the best ones and Earth is next. But why would they need so many Gems? With their level of technology, they’d decimate any race that stands in their way. A huge army is probably not needed when you can kill ecosystems just through reproduction.
My theory is that there is a bigger bad. A threat that the Gems desperately need a huge army for. Note that there is a non-Gem, non-organic presence in the show right now: the bismuth crystal inside Lion. Bismuth is a metal that can form crystals. Maybe there are sentient metals, and Rose Quartz encountered a scout of theirs, bubbled it, and hid it inside Lion. After all, she wanted to try to uncorrupt the Gems in the Bubble Room, if a non Gem was in there, it would just raise questions, and if it got unbubbled because of those questions, then it could get away and be like “I AM REPORTING THIS”.
Now these sentient metals, who could they be led by, and how much of a threat do they pose? I would assume either the strongest or most valuable metals would be leaders. Titanium or tungsten for the former, Gold, platinum for the latter. And instead of killing planets, like the Gems (though mining a planet of all of the metals would be a comparable method), they kill stars. Stellar fusion makes metals up to iron, heavier metals are made by supernovae.Â
Also, the metals could alloy. Gold + silver = Electrum, Copper + tin = Bronze, or my favorite: Tungsten + Titanium = Tungstanium.
For an added dose of horror: Gems cannot regenerate if the are set in something. What do we, as humans, set gemstones in most of the time? METAL. Victorious Metal warriors could set Gems into their own bodies as trophies of war.