"I think anybody could do it," he says, which is always a weird thought. Sometimes he wonders how Gems could have been on earth for thousands of years without it ever happening, but who knows. Maybe it did and the baby just... hm. "As long as... they could shapeshift the right stuff."

Steven only has a vague notion of what the right 'stuff' is, but presumably biology is involved, which Gems don't normally have.

"But it'd be kind of weird, right?" he asks a moment later. "Wanting to do it so much that you'd just stop existing..."

He can't imagine any of his other Gem friends wanting to do that. Why would they? Why did she?