The Cross Blast supercharges a Jelly causing a huge cross shaped like explosion. Use it to remove Slime Guards, liberate Caged Jellys and hit Living Stones.
Buy it by tapping on its icon at the bottom of the screen. Activate it any point in the game by tapping on it and choosing a Jelly on the board.
Tip: Use the Cross Blast to easily bring a Diamond down the board.