Lets not get hyperbolic. I found a real weapon with less metal than my jeans in about 2 minutes with google.
http://www.flickr.com/photos/isotope/492829645/
(Presumably a terrorist could do it with no metal, but a prisoner has limitations on materials available.)
A wooden mallet with tapered ends could easily crack a skull or break joints and limbs if used properly and would be easily concealable compared to a high density knife blade.
Considering that a war hammer is still likely the most dangerous weapon to an armored soldier today, there are worlds of weapons that are not routinely used or thought of that could be deadly in the right circumstances.
Heck, a well-trained person's hands can be quite deadly to someone not wearing armor... and if you know what you're doing, armor won't stop you from ripping someone's arm apart and spiking them into the ground with their own weight + that of the armor -- and yours, if you're feeling particularly aggressive :)