Please remember that certain skills are just natural upgrades of existing skills in order to keep skill bloat down. Look at Thunder I, Thunder II, and Thunder III. All three have the same effect. The only difference is the increasing damage and increasing cast time. Thus, each one can technically replace the previous one on the skill bar in order to minimize skill bloat (yes, for the anal mix-maxers you should always hard-cast Thunder II and only use Thunder III when the proc occurs, but for 99.9% of content that kind of min-maxing isn't going to matter at all).

Stone III can replace Stone II in a similar way. It doesn't have to have it's own special effect. (though it would be cool if Stone III had a proc that made the next Aero II/III instant to build more synergy between different skills).