Edict
Thousands of years ago, Pink Diamond gave the Grand Diamond a cut so deep she nearly shattered. Diamond did not, instead she fled to Earth with her most loyal of followers, unable to to fight back, weakened by her gem's imperfection so that even a mere Jadeite could hold her own.
The flaw, Diamond eventually decided, was not in her gem, but in herself. She was too old, her weakness too well known, what needed to be done was to reinvent herself.
So she found a human she liked, perhaps loved, and she had a child with him. She gave the child her gem, and she disappeared from the world. She left those loyal to her with a handful of last words, that seemed at the time like a prophecy.
"He will be Steven, the greatest of all humans and all gems, and he will defeat my enemies."