Black TUSQ XL is a proprietary material which is precision engineered under high pressure and heat. It is impregnated with PTFE throughout so it reduces the friction between the nut material and the string.  It is specifically formulated to deliver the right frequencies with the optimum transfer rate to the top of your guitar every time.  Because of the way Black TUSQ XL is made, it is a consistent material that is rich in tone and harmonic content.