Gittin 2:3

A get may be written with any substance: ink, dye, paint, resin, vitriol or anything else that is permanent. One may not write a get with beverages, fruit juice or anything impermanent. One may write on anything, such as on an olive leaf, on a cow’s horn – and he gives her the cow – or on a servant’s hand – and he gives her the servant. Rabbi Yosi HaGlili says one may not write a get on a living being or on food items.

Gittin 2:4

A get may not be written on anything that is attached to the ground. If a man wrote a get on something that was attached to the ground, then he detached it, signed it and gave it to his wife, it is a valid get. Rabbi Yehuda says a get is not valid unless it is both written and signed on something that has already been detached. Rabbi Yehuda ben Beseira says that one may not write a get on paper that has been erased or on unprocessed parchment because they can be tampered with but the Sages permit such documents.