There's one hidden in every second act. Activate it by standing on top of it, then beat the level. You'll go to a side stage, that has a gimmick of sorts most of the time. The emeralds will be at the end of these levels. Be warned, the temples are NOT easy to find.....they're well off the beaten path, and some are hidden under tiny ceilings that you have to spin under. Even I had trouble finding the one in Sunken Plant. So, good luck! If you need help, I'll be willing to help out.