I'm going to invade my roommate's privacy to take back an item I owned
What item did you roommate have?
It is something that my great grandmother gave me and is special to me.
Wanting what is yours is nothing to be ashamed of. Invading someone's privacy is not the best solution but you have a point in getting the item back somehow. Just be sure your roommate doesn't think you are stealing from him, he can call the cops and then there will be a court case.
I won't let people steal from me.
I understand that and you have every right to take it back, but I think you should ask first. If they don't give it back, get the police involved, but don't potentially harm yourself by invading their privacy.