Gem bombs are available in the Gemcraft series. They can be used for many things including killing monsters, enraging waves, destroying buildings, and damaging towers and beacons. In Gemcraft Chasing Shadows they can be used to decrease banishment cost of monsters. The gem bomb will spread out the damage (depending on it's grade) evenly upon enemies as well as the Gem's special. (For example, a grade 1 Armour-tearing gem does 60 splash damage and tears 2 armour, if two Reavers were to be present in the gem bomb's vicinity, each Reaver would take 30 damage and 1 armour.
When a gem bomb is dropped on a wave, it will enrage the wave making it stronger and worth more xp. A gem bomb will increase health, armor, experience, number of monsters, mana worth, and mana banishment costs.
Gemcraft Chasing Shadows
When 5 gem bombs are dropped on a wave, it will summon a beacon at the start of the wave. This is only in Gemcraft Chasing Shadows.