Not to be confused with Crystal Bell.
The Crystal Ball is a treasure found in "Jailbreak" of Sly 2: Band of Thieves. It is booby-trapped, therefore it has to brought back to the safe house before time expires. Once there, it can be sold on ThiefNet for 504 coins.
It is a large ball of crystal sitting on a shaped gold or bronze base. Attached to the front is the booby-trap. It is found inside a fenced-in area across from where Sly Cooper starts the "Code Capture" job.