It actually does tell you the whole time about the gem bomb thing. You just have to hover over the gem bomb spell and it shows you all the things it can do.
It's the exact same concept as nanowar. You put a different theme to it but other than that it's nano war. Makes me sad. 1/5 for makin it look different.
 "GemCraft chapter 0"){kind=icon}
 "The Idiotic Pop Quiz"){kind=icon}
 "Chronotron"){kind=icon}
 "Mechanical Commando"){kind=icon}
 "- Music in Motion -"){kind=icon}
 "Elite Forces: Clone Wars"){kind=icon}
 "Snake-roid"){kind=icon}
 "Battle Fish"){kind=icon}
 "Johnny Rocketfingers 2"){kind=icon}
 "Dark Cut"){kind=icon}
 "Darkness Episode 1"){kind=icon}
 "Meat Boy"){kind=icon}
 "Grid16"){kind=icon}
 "Red"){kind=icon}
 "Ether War"){kind=icon}
 "ELEPHANT RAVE"){kind=icon}