Gem
In Wonderland Adventures and Mysteries of Fire Island, Gems are bonus items that the player can collect. All gems present in the level, along with all coins, must be collected if the player wants to get Gold.
Gems come in two shapes: one is a small diamond-shaped stone that gives 100 points, and the other is a larger, more flat-shaped stone that gives 250 points. The latter shape was not seen in Wonderland Adventures (except in a sidequest), and it only became known to the public (minus the players who did the sidequest) once the Wonderland Adventures Editor came out.
The Rainbow Shards
The Rainbow Shards can actually be placed in the editor as gems, although they act differently than gems: instead of adding to your score, they go into your inventory as items. However, unlike the Rainbow Shards in Wonderland Adventures, they do not serve a functionality in custom adventures.