Avalon Code

I'm fucking stuck in a part.
You go to the forest, and there's a door what just opens if you equip "FRST" on your key. Alright, I do that and the door opens. I kill the monster and flip the switch, but appears something like a cristal or emerald. My question is: what I have to do with that cristal?

Break it with your FRST sword.
http://www.gamefaqs.com/portable/ds/file/946197/55967

Replying to #2

Thank you, you've helped a lot.