bone. There are also Teflon bone folders, but they are a tad expensive if you are working with a tight budget. Teflon folders from Gaylord will cost roughly $35.00 for the smallest size.
But what is a bone folder? According to Wikipedia, "A bone folder or bonefolder is a dull edged device used to crease material in crafts such as bookbinding, card making and any other craft where a sharp crease is needed. Often made from the leg bone of a cow, deer, or similar animal (hence its name). There are also synthetic alternatives available made from plastics. Bone folders made of Teflon have the advantage of not creating shiny marks on paper during use" (Wikipedia, Bone_folder).
Learn more about the origins of book binding from Wikipedia.