Baseline features include:>a Gem to provide sustainance to the creature>aesthetic decisions that indicate the species that made them wasn't short resources but wanted a beautiful servant race - mouth, hair, clothes, voices, gems looking like gems, all that stuff isn't necessary to an automatic colonizer race but they have it nonetheless>the animalistic loyalty to the leader is greatly enhanced to the tune of being ready to die for the cause/master>a built-in simple system for identifying competent and capable gems - cut, color and such features
In essense, the Gems are an artificially uplifted animal species with two specific changes turning them into gems we know and masturbate to.
Change one: physical alteration. That is what keeps a 'corrupted' gem from manifesting as they looked like in their animal days, instead turning into varied beasts.
Change two: mental alteration, or 'programming'. It's a robotic set of instructions, guidelines, rules, codes and morals gems are baked with. The smarter is a gem, the more coherent it will be, the more competent, the stricter obeyer of the programming and the less will it act out in emotion, and vice verca. Emotion is disallowed among gems and repressed not because they don't have it by default and are robots, but because it ruins their programming and lowers their productivity. Fusion is strictly controlled and distrusted because fusion leads to emotion. You keep them fused too much and before long they go TOPAZ LIKES TOPAZ and half your fucking court is crying instead of doing their jobs.
I had outlined this theory a while ago, but this is a summarized version. It also relies on the gems having a single precursor species instead of several, which would explain corruption as them losing control of their form to the tune of reverting to the old one.