Gemara

The Torah does not require a knife for slaughter.

(Mishnah): If one slaughters with a hand scythe, rock or reed, the Shechitah is valid.

Chulin 15b (continuation of Mishnah): One may slaughter with anything, except for a sickle, saw, teeth, or fingernail, for these tear the Simanim.

Question: 'If one slaughters' connotes b'Di'eved. Granted, one should not Shechitah with a hand scythe, lest he come to slaughter with the side that is not smooth. However, a rock or reed is Kosher l'Chatchilah!

(Beraisa): One may slaughter with anything, e.g. a rock, glass, or a reed.

Answer: It is b'Di'eved when they are attached (to the ground). When they are detached, it is l'Chatchilah.

Rishonim

Rif and Rosh (Chulin 4b and 1:21): One side of a hand scythe is a (smooth) knife, and the other side is a (jagged) saw. One may not slaughter even with the smooth side, lest he slaughter with the jagged side.

Rosh: Therefore, if a knife has a nick, one may not slaughter even with the smooth part, lest he use the nicked part. On Yom Tov, the custom is to allow tying a cloth around the nick (to ensure that it will not be used to slaughter), for one may not sharpen the knife. One who slaughters with a double-edged knife must check both blades.

Ran (DH Gemara): When there is a reminder not to slaughter on the part with the nick, one may slaughter l'Chatchilah. We forbid with a hand scythe, for that is the form of the Kli; so perhaps people will err (and slaughter with the jagged side). Here, it is an incident that there is a nick; we do not decree.

Rambam (Hilchos Shechitah 1:14): One may slaughter with anything, e.g. a metal knife, rock, glass or reed, or similar things that cut. This is if the cutting surface is sharp, without a nick. If there was like a furrow on the edge used to slaughter, even if it was very small, the Shechitah is Pasul.

Magid Mishneh: 'Things that cut' excludes tin, lead and similar things

Poskim

Shulchan Aruch (YD 6:1): One may slaughter with anything detached, e.g. a knife, rock, or swamp reed (Rema - or tooth or lone fingernail) or similar things that cut. This is if the cutting surface is sharp, without a nick.

Rema: One may not slaughter with other reeds or glass from which chips fall off and there is concern lest the Simanim get punctured.

Beis Yosef (DH b'Chol): Ba'al ha'Itur says that since this is the concern, even b'Di'eved it is Pasul. However, according to the Yerushalmi the concern is Ru'ach Ra'ah, so b'Di'eved it is Kosher.

Taz (1): If chips fall off, the cutting surface is not smooth! It seems that the Rema teaches that even if after slaughter we see that it is smooth, the Shechitah is Pasul, for perhaps a chip came off and punctured the Siman.

Question (Taz 1): Why does the Yam Shel Shlomo say that this is only l'Chatchilah? Ba'al ha'Itur disqualifies even b'Di'eved!

Answer (Shach 3): It is Pasul b'Di'eved only if the reed was lost. If we check and see that chips did not come off, all agree that it is Kosher!

Machatzis ha'Shekel: If a regular knife was checked before slaughter, if it was lost afterwards, we rely on Chazakah that it remained smooth. There is no such Chazakah for other reeds.

Pri Megadim (Sifsei Da'as 3): According to the Shach, we can say that the Yerushalmi merely adds a concern for Ru'ach Ra'ah even if the reed was checked afterwards (so the Shechitah was Kosher).

Bach (3 DH v'Chosav): It seems that the Bavli argues with the Yerushalmi, for Rav Papa and Rabah bar Rav Huna used other reeds to cut fish innards and soft fowl, in a way that there is no concern for chips. They were not concerned for Ru'ach Ra'ah! However, perhaps they cut cold fish and fowl, and Ru'ach Ra'ah applies only to something hot, like the place of slaughter.

Taz (1 and Shach 2): Why is the Rema concerned for chips from glass? The Gemara is concerned only for reeds! I did not see any Posek say that there is a concern for glass. Regarding Milah, the Rema forbids reeds from which chips fall off, but does not mention glass. It seems that there is a printing mistake; 'glass' should be in the words of the Mechaber, not in the Rema.

Shulchan Aruch (ibid.): If one side of a knife is a saw, and the other side is good, one may not slaughter with the good side l'Chatchilah. This is a decree lest he slaughter with the other side. If he slaughtered with the good side, it is Kosher.

Rema: The same applies to a long knife with a nick, and the smooth part is long enough for slaughter.

Shach (5): If one goes back and forth, the blade can be miniscule! Here we require a blade long enough to slaughter in one motion (twice the length of the neck). If he goes back and forth, perhaps the cloth will fall off.

Rema: One may not slaughter even with the smooth part, even if he tied a cloth around the nick. If he slaughtered and says that he is sure that the nick did not touch (the place of slaughter), it is Kosher, even if he did not tie a cloth.

Taz (3): He is believed only if he knew about the nick when he slaughtered.

Rema: On Yom Tov, when one may not sharpen the knife, or during the week in pressed circumstances, it is permitted if he ties a cloth around the nick. L'Chatchilah one should be careful not to slaughter even if it has a nick only between the handle and the blade.

Beis Yosef (DH u'Mah she'Chosav uv'Yom): The Kolbo says in the name of the Rif that even on Yom Tov it is permitted only if the smooth part of the knife is twice the length of the animal's neck. However, the Ran and others permit l'Chatchilah even on a Yom Chol if one made a reminder. The Roke'ach says that one should not do so, lest the garment fall off. L'Halachah, one may rely on the lenient opinions in pressed circumstances.

R. Akiva Eiger (1): Tevu'as Shor (24) forbids wrapping in a way that covers a greater length of the thick side of the knife than of the blade. If he cuts in a place of the blade opposite a covered part (of the thick side), this is Chaladah (slaughter with a covered knife, which is Pasul).