Board Thread:General Discussion/@comment-4224349-20150205230904/@comment-26082492-20150214094109

The fact that Steven and Amethyst walked to the kindergarten location blew my mind and convinced me that all the gem's missions have been on earth.

The gems themselves have stated that they can't warp outside of earth. They destroyed the Galaxy warp in order to keep the other gems away.

The mirror that imprisoned Lapis Lazuli was found on the Galaxy warp area, at least according to Pearl, and Lapis blamed the crystal gems for trapping her in it. The gems acted as if they didn't know what the mirror actually was so I suspect Rose trapped Lapis in it, without telling the others,  to keep her from using the Galaxy warp to leave. They then sabotaged the Galaxy warp.