Board Thread:General Discussion/@comment-28310993-20150625224552/@comment-26289669-20150625225822

Garnet: First you need a gem at the core of your being. Then you need a body that can turn into light. Then you need a partner you can trust with that light.

Greg: Metaphorically?

Garnet: Literally.

Steven meets all three criteria required to fuse, and since he is half Human he can fuse with Connie, someone he trusts with his light.