Knowing no one likes reading instructions, I've tried to be as curt as possible for Unicopia, resulting in 13 words describing how to play. 

However, for players that still clicked through the instructions, it wasn't too obvious that the gems needed collecting, and failing to collect then makes it impossible to complete a level.

I therefore took a note out of The Design of Everyday Things and added some sparkle particles to the gems as a signifier to the player.  Adding in a little wobble and some SFX when the player approaches, the gems are now much more in the player's face, but is too much?