Talmud for Gittin 2:3
בַּכֹּל כּוֹתְבִין, בִּדְיוֹ, בְּסַם, בְּסִקְרָא, וּבְקוֹמוֹס, וּבְקַנְקַנְתּוֹם, וּבְכָל דָּבָר שֶׁהוּא שֶׁל קְיָמָא. אֵין כּוֹתְבִין לֹא בְמַשְׁקִים, וְלֹא בְמֵי פֵרוֹת, וְלֹא בְכָל דָּבָר שֶׁאֵינוֹ מִתְקַיֵּם. עַל הַכֹּל כּוֹתְבִין, עַל הֶעָלֶה שֶׁל זַיִת, וְעַל הַקֶּרֶן שֶׁל פָּרָה, וְנוֹתֵן לָהּ אֶת הַפָּרָה, עַל יָד שֶׁל עֶבֶד, וְנוֹתֵן לָהּ אֶת הָעָבֶד. רַבִּי יוֹסֵי הַגְּלִילִי אוֹמֵר, אֵין כּוֹתְבִין לֹא עַל דָּבָר שֶׁיֵּשׁ בּוֹ רוּחַ חַיִּים, וְלֹא עַל הָאֳכָלִין:
Gittin can be written with all things: with ink, with paint, with sikra [a red dye], with gum resin, with vitriol, and with all (other) things whose impression remains. They are not written with juices or with fruit-liquid, or with any (other) thing whose impression does not remain. Gittin are written on all things: on a [torn-off] olive leaf, on the horn of a cow (and he gives her the cow) [for he cannot cut the horn off after he writes it, it being written (Deuteronomy 24:1): "And he shall write her a scroll of divorce, and he shall place it in her hand" — that which lacks only writing and giving; to exclude that which lacks writing, cutting and giving]; on the hand of a bondsman, and he gives her the bondsman. R. Yossi Haglili says: Gittin are not written on things which have a spirit of life, and not on foods. [For the Torah called a get "sefer" (a scroll). Just as a scroll is characterized by not having a spirit of life and not being edible, so, all that does not have a spirit of life and is not edible. And the rabbis say: if it were written: "in a sefer," it would be as you say. But now that it is written "sefer," sefirath devarim ("relating details" of the get) is the intent. The halachah is in accordance with the sages.]