The Infinity Stones would be Gems themselves, progenitors of the Diamonds and the source of their powers and abilities, perhaps forgotten with time. The individual Gem types would each more closely align to one of the stones in terms of what they could do, but extremely limited in power and scope. The reason for the taboo against cross-gem fusion would be in part to prevent the Gems from multiple factions from combining their powers because in theory you could create a being that would have the powers of the Infinity Gauntlet on a smaller scale, which would still be dangerous. The Gems themselves as a race would turn out the be the divided consciousness of the seventh Infinity Stone, the Ego Stone which was the consciousness of the primal omnipotent being that the Stones originally came from. At some point, the Ego Stone would seek to reconstitute its self by absorbing the entirety of Gemkind but is unable to compel Steven to merge with it due to his dual human/gem nature. In the end, he chooses to merge with Ego but in doing so his presence in Ego’s being allows her to appreciate the nature of individuality and friendship which convinces her to divide herself up once again to restore the Gems to their original state. Steven goes on to join the Guardians of the Galaxy.