Jade had a Magic Cue Ball in her chest in the beginning -- p=002703 is when we first see it, then she picks it and the Magic 8 Ball up and there's some exposition on it on p=002705. This is the first time the Magic Cue Ball shows up in the comic, and it is likely that Rose later came into possession of it, since I don't think there are very many of them in existence, and Mindfang/Vriska's exploded.

Bonus Fact: Doc Scratch once refereed to the Magic Cue Balls as his seeds.

~Mod T