They said it couldn't be done! 6 gems in a row!
This achievment is luck based! Playing Diamond Mine is the easiest way to create a supernova gem because it is the fastest mode and has the most special gems per minute. You can try to line up gems and hope to get the right match to drop them but it is way faster and easier to just play a few rounds of Diamond Mine without paying attention to supernova gems. Create cascades and huge explosions to clear the bigger part of the screen and you'll create a supernova gem if you are lucky. It took me a handful of games before I got lucky and created a supernova gem. Lining up 6 gems in a L-form does NOT create a supernova gem (it creates a star and a flame gem) - they have to be right next to each other in a straight line!