J'ai trouvé ceci, j'espère que ça pourra t'aider 
"Cascade Stones - The Cascade Stone on Stage 7 is one of the most difficult, requiring a Cascade x7. This video shows a Cascade x16 on Bejeweled Twist, the game on which Gyromancer's puzzle system is based. Below it you can find a diagram showing specifically a Cascade x7.
In order to utilize this method for only a 7 Cascade, start building it in the bottom left corner of the field. While you can of course use any color types you desire, it can help to match them to the screen (sans the extra type in Bejeweled Twist of course) to keep better track. You want to avoid mixing up colors too much, so that you don't have an unexpected combination that voids the rest of the cascade.
Here is an additional diagram showing another method to get just a x7 Cascade for those who prefer.
http://img134.imageshack.us/img134/9530/7xcascade.th.jpg
Bon courage 
