Gems randomly appear and you can't predict when and where one will turn up.
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the way I got it was, I went to France (World Adventures) then I explored many tombs then finally I found it!
You have to hack the game cause the rainbow gem was unfinished. so hacking is the only way to get it.
you cant.
instead of hacking complete all town and act missions with s rank on silver sonic and shadow and you will have the gem
Howling Horse Mine, Muzzle Mouth Mine and Buried Bones Mine.