It's naturally only a theory based on what the crew said and what the show said, what we know about the Gems and what we don't.
Facts:>Gems are a technology, not a naturally occuring species>all Gems are artificially made, none of them came to being artifically>Gems have a great tendency to act animalistic when in an unexpected situation, i.e. Peridot's gremlin behavior as she was unbubbled>Gems greatly favor lack of emotions and robotic behavior over emotions despite their emotional nature and animalistic tendencies>White Diamond, the tyrant of the gem race, backed down after getting knocked down on her ass>The Gem Empire pointlessly favors devouring worlds with life on them rather than focusing all other worlds, and that's in a galaxy in which life is very rare, especially intelligent life>Ronaldo is typically mostly right excluding a few details, and his claim is that a reptillian race is in control of a society (human one as per him)
This allows me to utilize my powerful autism to write up a theory of Gem origin, purpose and nature.>the artstyle has the gems' teeth be typically sharp with a few exceptions being undetailed wide smiles, but the sharpness is most prominent when gems need to be intimdating/show anger
Gems originally were an inorganic (be it silicon-based or some sort of dense light form, however exotic that sounds) animalistic lifeform - perhaps one with shapeshifting abilities to a certain small degree. They probably didn't have gems in the form we see it in the show. They were some sort of inorganic predators, maybe, requiring sustaincance still, with very firm wolf pack-life hierarchy. What the top dog says, goes. Strongest gem gets to be the top dog.
An unknown, possibly humanoid, but most likely snake-like species came upon the world. From then on the Gem precursor species was transformed into what we know as Gems today.