Look at these shapes and materials!
Historically we've made them out of brass, glass, hard polished wood, iron, porcelain... probably some other stuff that I'm forgetting, too.
Doorknob materials had to be very durable & stand up to a lot of use & handling, but still be easy enough for the doorknob craftsman to work with and form into a nice smooth shape that would feel good in your hand.
Antique doorknobs can be amazingly decorative. They even have fan clubs.