If a valuable item of mine was given to someone else without my knowledge, do I still have the right to ownership?

If the item is rightfully yours your roommate can not give it away in order to settle his own debts. Under property laws, it is your personal property. You are still entitled to ownership as long as you did not abandon the item (leave it with your roommate making it clear you were not coming back for it) or give it away to cover your debts. It does not sound like either of these actions occurred. If this is the case you would be entitled to replevin which is the legal remedy for a person to recover goods that have been unlawfully taken from their possession. This is a speedy process of regaining possession of an item if peaceful self-help (asking for it back) has not worked. A writ of replevin is generally filed by the court and served by a law enforcement officer. The requesting party may be required to post an indemnity bond. All service and bond requirements are generally state specific therefore I would contact a local attorney or speak to a local court clerk in order to initiate the process.