This isn't as hard as it seems. As you can make this very easy for yourself.
When you start on the first level, you will see some very colourful magic glasses. Pick these up, and you will notice some twinkling stars dotted about the level. Shoot these and Gems will fall from their invisible hiding place.
If you die however, you will lose them. If this happens, look for a door in the levels with a Shop sign above. Here you can buy them, and continue searching them out. It really doesn't take too long to get 100 found. Happy hunting.