There are 6 different types of Gems in this game.
- Fire (Red)
- Water (Blue)
- Earth (Green)
- Light (White)
- Darkness (Purple)
- Life (Yellow)
Occasionally some quests will have an X over one of the gems. That means that the dungeon will not spawn any of those gems. (As a helpful tip, don't bring divers of that type usually).
Elements also have weakness and strengths. When using a elemental attack that is strong against the enemy, you will do 2x more damage. Likewise, if you use a weak elemental attack you will only do 0.5x.
Incase you forget which element is weak to what, there is a handy option to include the elemental triangle on your map.
As you can see Fire > Earth > Water > Fire. Both Light and Dark will do 2x to each other.