Wikipedia
GemCraft is a tower defense flash game created by Game In A Bottle, originally released June 26, 2008. A prequel, GemCraft Chapter 0: Gem of Eternity, was released April 16, 2009, and the sequel, GemCraft lost chapter: Labyrinth, was released February 17, 2011. The original was released for iOS on April 14, 2011 by NTT Resonant.
In GemCraft, players are given gems to combine and put in towers. Different combinations of gems produce different effects, such as splash damage, Damage over time and triple damage. The player's life bar and magic energy are represented by the same statistic - mana. With mana, the player can purchase towers, gems, trenches, traps, and combine gems. A key aspect of strategy in GemCraft is to increase the player's mana gain to high levels, enabling creation of stronger and stronger gems. Combining two gems creates a single gem of higher power, or 'level'. The gems change with level.