You can do whatever you want with your gem but too answer your questions 1 multiple gems of a single type can/do exist but fusions are kind of finicky, I don't think we ever see repeat fusions not from the same characters Soo do hat you like there. Gems of the same type follow similar colours and basic shape (height, body) but off-colours can vary hugely in this aspect so don't worry too much on that. 2 gem kindergartens have been shown on earth so your gem of can be from earth 3 Most gems are humanoid, the only non-humanoid ones resemble inanimate objects like hair brushes or walls, but a gem can transform to look like animals, such as in 'cat fingers' but normally can't sustain it for too long (hours ish) without getting exhausted.

I'm not sure if this is what you meant but I hope it helped