Nuts, Saddles, and Bridge Pins
The nut and saddle (and to a lesser degree the bridge pins) must be made from a hard material that allows string vibration to pass to the soundboard without loss or coloration. Ivory is nearly perfect and has been used for centuries. Sadly, ivory poachers have decimated the elephant population to near extinction, and legal (pre-ban) ivory is hard to get.
Fortunately, cow bone looks just as good, and is very good acoustically. VanDruff guitars come standard with bone saddles and nuts.
For those who like the look, I can also outfit your guitar with fossilized mammoth ivory. Mammoth ivory isn't really fosilized, but it has been buried for eons and has picked up a mineral stain that gives it a unique look. It is also extremely hard and may result in increased volume and a brighter tone than bone or ivory.
Bridge pins can alter the sound of a guitar too, but the effect is subtle. Although ebony wears too fast to make a good saddle, it makes excellent bridge pins. I like ebony and bone equally well and choose the one that best compliments a particular guitar.