Jasper said that she respected Rose's tactics what if Steven was one of them. This theory is that Rose made a tactical decision to make Steven. In "Jail Break" Steven was immune to nearly everything on the ship that was able to hold normal gems. Her decision to have Steven was so that the gems would have a secret weapon that new gem technology would have no effect on. Steven is like a super soldier for the gems. This is also supported by the fact that Rose was such a strong and capable leader why would she give up her physical form and leave the gems to their own defenses.