Before us is Mishnah 3 in the second chapter of Maseches Gittin. This Mishnah deals with two matters: which kinds of ink and dye may be used to write a get, and on which materials it may be written.
The Materials With Which One Writes:
The Mishnah opens and says: "B'chol kosvin" - a get is valid when written with any type of ink:
"B'dyo" - ordinary, standard ink, made from gallnuts (the growths that form on a tree).
"B'sam" - ink made from a certain type of red earth.
"B'sikra" - another kind of red dye.
"U'v'kumos" - made from the sap of a tree.
"U'v'kankantom" - copper sulfate.
"U'v'chol davar shehu shel kayama" - and likewise with anything that has a permanent, lasting quality.
The Materials Invalid for Writing:
In contrast, the Mishnah continues: "Ein kosvin lo b'mashkin v'lo b'mei peiros v'lo b'chol davar she'ein bo miskayem" - one may not write with plain liquids, such as grape juice or berry juice, nor with other kinds of fruit juices, nor with anything that does not last. The reason is one and the same: none of these have permanence and durability.
On What Is the Get Written:
Regarding the material upon which the get is written, the Mishnah rules: "Al hakol kosvin". One may write even "al he'aleh shel zayis" - on an olive leaf, provided that it was detached from the tree and is not attached to the ground. For the verse says, "and he shall place it in her hand," and therefore it must be something that can be detached and given into the woman's hand.
So too one may write "al keren shel parah" - on the horn of a cow, "v'nosen lah es haparah" - and he hands her the entire cow. However, one may not cut off the horn after writing on it and give her the horn alone, because the verse says, "and he shall write for her a document of severance and place it in her hand": the writing and the giving are adjacent to one another, and there is no room for an additional act in between. The cutting constitutes an interrupting act in the middle, and it therefore invalidates the get.
Similarly, one may write "al yad shel eved" - on the hand of a slave, "v'nosen lah es ha'eved" - and he hands her the slave himself, for the same reason: one does not detach the writing from its place.
The View of Rabbi Yose HaGelili:
Rabbi Yose HaGelili disagrees and says: "Ein kosvin lo al davar sheyesh bo ruach chaim v'lo al ha'ochalin" - one may not write on something that possesses life, neither on the horn of a cow nor on the hand of a slave, and likewise one may not write on something considered food. Even an olive leaf is included in this, since it is eaten by doves and is considered animal food.
In summary: In this Mishnah we learned that a get is valid when written with anything lasting - dyo, sam, sikra, kumos and kankantom - and invalid when written with liquids, with fruit juices, or with anything that does not last. We also learned that one may write on anything, provided that it is handed to the woman as it is, without any act of detaching or cutting between the writing and the giving; and according to Rabbi Yose HaGelili, one may not write on a living creature or on food.