Lets face it, the handle is about half of the knife. A good knife handle has to be comfortable and preferably strong and durable. A waterproof handle is also a big plus. Aesthetically, the handle is where a knifemaker is allowed a lot of creativity and wide variety of materials, or combinations of materials can be used. I prefer to use natural handle materials but I do occasionally use synthetic man-made materials for my knife handles. Sometimes it is a combination of both as natural wood and bone is often "stabilized" with synthetic resins to prevent shrinkage and cracking over time.
Rare and exotic handle materials can be very expensive. Bear in mind when considering the expense of a handmade custom knife that the handle material alone can cost well over $100 in some cases.
Stabilization: Before we get into materials, I think it is important to mention that many natural materials including bone and many types of wood can be "stabilized". Stabilization is a method of injecting resins into the material under vacuum. The resins make the material harder, more waterproof, and more stable against shrinkage or cracking. Colored dyes are often mixed with the resins to achieve incredible patterns and colors. Softer woods are often stabilized for use in knife handles. Some hard woods such as ironwood and ebony cannot, and do not need to, be stabilized.
In fact, there are so many options that I have broken this section into separate parts dealing with different types of handle materials: