A get can be written with any substance that lasts - and the mishnah provides a list of them. A get can be written on any substance too - with a dispute over whether that includes living creatures and (lehavdil) slaves. Plus, R. Yehudah, who took great pains to read and sign his own documents, even when he could have managed otherwise. Also, when confusion creeps in to the delivery of a get - depending on the shaliach, on a throwing by the husband, etc. Plus, a whole lot of unusual cases.