Things in the Game: Diamond
This square contains a diamond. Rich needs to collect all of them for the exit to open. To collect a diamond, simply move Rich into the square containing the diamond.
If a puzzle has covered squares (these are rare), a diamond or two might be hiding in those squares. The status bar will tell you how many diamonds are left on the level. If you don't see any diamonds, but the exit has not opened, check the status bar to see how many diamonds are still hiding.