Tone Ninja nuts are made from an engineering polymer (more about that later) that was specifically chosen to meet the specific characteristics required of an outstanding nut for your guitar - namely high density (to transmit tone), low friction (to avoid sticking and binding), and easy work-ability (to allow perfect custom fits) while retaining wear resistance (so it doesn't wear out).
Originally, nuts were traditionally made of bone, carved and slotted to suit. The main problem with bone is that it can be hard to work, and can vary in density. Alternatives were various forms of ivory, which are now, of course, ecologically unacceptable.
As technology progressed, various different materials were used, including brass, graphite, low cost plastic (still used on many entry level instruments and even some higher priced guitars), engineered plastics such as micarta, corian and Graph Tech's Tusq material (which is actually an injection molding plastic called polyphenylene sulfide).
When we were selecting a material for Tone Ninja nuts, we wanted to improve on these materials rather than just imitate them, and as a result selected an acetal copolymer with glass fiber reinforcement (For the technically minded, more information here) because of it's high stiffness, low coefficient of friction and excellent dimensional stability.