Radiant Emerald is the fifth and final course available for play in Sonic R. It is the only course which has to be unlocked, which requires coming in 1st place on the other four courses in the Grand Prix mode.
This is a magical rainbow course surrounded by crystals that swirls around planets through outer space, similar to the famous Rainbow Road courses from the Mario Kart series. The Ring Gates here contain massive shortcuts, which can be used to the racers' advantage. Of all the courses in the game, this is easily the longest and most straightforward of the lot, and even if the players open the shortcuts, completing one lap will still take a while.
It is the only course to contain no Chaos Emeralds or Sonic Tokens. By default, this course's theme is "Diamond in the Sky". However, if a player uses Super Sonic here, "Super Sonic Racing" will play instead.
||Hey, what's the X on the screen for? Could it be a hidden course?
— Sonic R US PC manual