The standard nut on PRS guitars is made from a specially-formulated friction-reducing black plastic that Paul Reed Smith jokingly referred to in an interview as "unobtainium."
“My Sales Engineer is the best. If I need it tomorrow, I have it tomorrow. He makes sure everything is in order and that I'm set up right. I've spent a lot of money at other places, too, but I don't remember them calling me to make sure everything's okay.” Nick Moon, Portland, OR