>>102145431He is part Greg though. Her species isn't created for reproduction but because of the shapeshifting power she was capable of it, however her physical form which is the gem cannot divide, so it combined with Greg to make a new life form. He is by all definitions her progeny, it simply doesn't work the same way it does for humans. Think of it like of a combination of regular human reproduction with plant budding that has one undividable but not necessary to function part, Steven could exist without Rose sacrificing herself but he wouldn't have the gem part of her, only the part that was created by her humanoid form.
