Hello again. I'm back with another theory, this one is about who the first Gems created by the Diamonds were and also how the Gem caste system my have evolved over the years.
Firstly, You are probably wondering where the Diamonds come from, and it will be important so you can find my theory here as a comment on this thread here.
Now let's explore who the first Gems after the Diamonds were.
Short answer. I think it's the Pearls. They seem to be the closest to the Diamonds and (ironically, despite being slaves) seem to be the second (third or forth if you count Mixed-fusions and defectives) to have more individuality than any other Gem type.
Another thing I noticed with our Pearl is that, for a non-fusion, she seems to have a lot of abilities then most other gems as well. Making her a "Jack-of-all-trades" of sorts. She can fight and summon a weapon like soldier gems, build and fix stuff like a Peridot and Bismuth and has the intelligence and holograms of a Zircon.
Because of this I don't think Pearls were always slaves. I think that when the Diamonds (or maybe just White Diamond assuming the others didn't exist until after) founded Homeworld, the Pearls were made to just be the people for the diamonds (or White) to rule other. They each have weapons/tools they can summon at will, and the power of fusion to do more difficult tasks. They also have holograms to help plan and explain tasks to each other. They also have shapeshifting to change thier outfits and shape for many different scenarios.
Evolution of the caste system.
So the Pearls were the first Gems made by the Diamonds, but what about the others? The next Gems I think made were Bismuths. The Pearls, at the time, where good at multiple jobs, but as Homeworld expanded, It was decided they needed help. The Bismuths I think where made to do the more "heavy duty" jobs like building and possibly fighting. Rather then having one weapon/tool each like the Pearls, these would have hands that could fluidly change into any tool they needed. They would also be heat resistant.
Rubies my have been created next to replace both the Bismuths and Pearls as fighters. They would have summonable weapons like Pearls and be heat resistant like Bismuths. They would also have the power to make heat themseves to use offensively.
As Homeworld grew, more Gems where made. Other soldier castes were created to fill muliple military roles. "Elite" Gems like sapphires, Aquamarines, Morgonites and others where created to help run the ever expanding empire and have other useful abilities (like Sapphires future Vision). Lapis Lazulis where made once Homeworld had gotten so full they needed to terraform other planets to make collonies
By this time, the Pearls ​who had once run Homeworld were reduced to mere servants.