Gems will allow you to customize your gear towards how you play the game. Gems are categorized into three groups—red, yellow, and blue. Red gems modify max health, melee, or magic. Blue gems modify crit power, physical resists, or magic resists. Yellow gems modify crit, haste, or dodge.
Each piece of gear has four gem slots. The first slot is unlocked automatically, but each additional space will need to be unlocked with silver coins or gold shields. Each slot will hold a different category of gem. Gems can only go into a gem slot if the color matches. This means red gems must go into red gem slots and yellow gems must go in yellow gem slots. The final slot is white and will take any gem regardless of color.