Gemara

Look on the mountain. There is only (Aruch - not even) one Chilazon. Tomorrow, rain will fall, and the mountain will be full of them!

(Rava): The Torah mentions Yetzi'as Mitzrayim regarding Tzitzis to teach that just like Hash-m knew the true fathers to kill the firstborns, He knows who puts Kala Ilan (blue tree sap) on his garment and says 'it is Techeles.'

Menachos 42b (Rav Shmuel bar Rav Yehudah): To dye Techeles, we take the blood of the Chilazon and cook it with dying ingredients such as alum...

43a: To test Techeles, R. Yitzchak brei d'Rav Yehudah would bring alum, fenugreek juice and urine of a baby 40 days old and soak the Techeiles in the mixture overnight. If the color was ruined, it is Kala Ilan. If not, it is Techeiles.

Rav Ada taught that one bakes it in a potent sourdough of barley. If the color improves, it is Techeiles. If not, it is Kala Ilan.

Mar (a Chacham) bought Techeiles. It failed the test of R. Yitzchak, but passed the test of Rav Ada. He thought that it was invalid.

Rav Achai: Will you say that it is neither Techeiles nor Kala Ilan?! Rather, the two tests complement each other. If it passes the test of R. Yitzchak, it is Kosher. If not, we apply the test of Rav Ada. If it passes, it is Kosher; if not, it is Pasul.

44a (Beraisa): The Chilazon has the form of a fish. Its color is like the sea. Techeiles is made from its blood. The Chilazon comes up only once every 70 years, therefore Techeiles is expensive.

Megilah 6a: Zevulun was complaining about his portion. Hash-m said 'I will make your brothers need you, through the Chilazon.'

Shabbos 26a: Yogvim are people who hunt for the Chilazon from the hill of Tzur to Chaifah.

74b (Rava): For the Mishkan, they would tie and untie knots in nets to trap the Chilazon.

75a (Beraisa): If one hunts a Chilazon and bruises (or presses) it with his hands to extract its blood, he is liable only once (for trapping).

R. Yehudah is Mechayev twice (also for Dash, extracting the blood).

(Rava): Chachamim hold that Dash applies only to Gidulei Karka (things that grow from the ground).

Bechoros 38a (Mishnah): The following are Mumim in the eye... Chalazon Nachash...

38b: R. Shimon ben Yosi ben Lekunya taught that Chilazon is Nachash.

Rishonim

Rambam (Hilchos Tzitzis 2:1): Techeiles is wool dyed like Patuch blue, like the appearance of a clear sky during the day. Techeiles for Tzitzis must be dyed in a known way, so it will keep its beauty and not change. If it was not dyed this way, it is Pasul, even if it is like the sky, e.g. it was dyed with Istis, Shachor or other blackening agents.

Sefer ha'Chasum says that Patuch blue is stricken, and it leans a little towards back. RD"A says that is like it is mixed with it. (Both are brought in Likutim in Frankel Rambam.)

Rambam (2): To dye Techeles threads, we soak wool in plaster and wash it until it is clean. We cook its blood with Ohala (alum) and other ingredients like dyers do, so it will absorb the color. Then we put it in the blood of the Chilazon, a fish whose color is like Techeiles and its blood is as black as ink. It is found in Yam ha'Melach. We put the blood in the pot with cumin and similar things, like dyers do, and boil it so it and put in the wool until it becomes like the color of the sky.

Rashi (91a DH Chilazon): Chilazon is the sea worm from which we get dye for Techeiles.

Ha'Gaon R. Chayim Kanievsky, Shlita (Perusho on Maseches Tzitzis, cited in Meir Oz 9:2:3): Rashi calls it a worm because it is a small fish.

Rebuttal (Ramah 91a DH Omar): If it is from the sea, it is no proof to Techiyas ha'Mesim! Rather, here Chilazon refers to a spontaneously generating land creature.

Aruch (Chilazon): In Sanhedrin, Chilazon refers to a locust.

Kupas ha'Rochlim (Tiferes Yisrael's introduction to Mo'ed, Klalei Bigdei Kodesh DH Hinei): In Menachos 44a, Rashi says that the Chilazon comes up from the ground. In Shabbos 74b, he says that it is a small fish. In Sanhedrin, he says that it is a worm. The Yerushalmi (Shabbos 1:3) supports this, for it says that it has no sinews or bones. Also Bechoros 38b supports this (Chilazon is Nachash). However, if it comes from the sea, how does it prove that the dead will live? The Rambam says that it is found in Yam ha'Melach. This is in Yehudah's portion. The Chilazon is found in Zevulun's portion! Also, we say that they hunt for the Chilazon from the hill of Tzur to Chaifah, which are near the Mediterranean, near Zevulun's portion.

Ma'adanei Yom Tov (on Halachah 7 of Rosh Halachos Kilayim (in Nidah Perek 9),10): The Rambam uses 'Yam ha'Melach' to refer to salt-water seas.

Kupas ha'Rochlim (ibid., and DH ha'Klal): The Aruch says that the Chilazon has a shell on its back. However, snails do not resemble the color of the sky. Really, Techeles does not require the Chilazon. We require only that the color will not change after the tests that the Gemara gives. We do not wear Techeles, for we do not know the ingredients needed to test it. Also, I asked expert dyers, and none knew of a blue dye that will not change. This is why we use only the Chilazon, and it is lost from us.

Tiferes Yakov: His text said that if it passed only Rav Ada's test, it is neither Techeiles nor Kala Ilan (and it is Kosher). In our text, this was said incredulously. (Surely, it is either Techeles, or (Pasul, like) Kala Ilan.

Perush ha'Mishnayos (Menachos 4:1): Since we do not know how to dye the wool with Techeiles, we wear only white strings.

Tosfos (42b DH v'Simemanim): It is astounding that one may mix other ingredients with Techeles! Perhaps together with the ingredients it is called Techeles. Rashi explains that the Techeles is soaked in other ingredients.

Question: Untying is an Av Melachah because they untied knots in nets to trap the Chilazon. The Chilazon is found only in the portion of Zevulun!

Answer (Radvaz 2:685): The Chilazon is found in the sea. In the portion of Zevulun it comes up by itself onto the land, and they would gather it. After Yisrael were exiled, it did not come up. Nevuzaradan left some poor people to be Yogvim, i.e. to hunt the Chilazon. This implies that one must trap it. When they made the Mishkan, they were at Har Sinai, which is close to Yam Suf. This is why we did not need to say about the Chilazon like we said about the Tachash (that it existed only when they built the Mishkan - Shabbos 28b). Perhaps the Chilazon is still found today, just we do not recognize it or we do not know how to trap it. Also, we do not need it, for similar dyes are available, e.g. Istis, which does not pass even through ironing.

Medrash Tanchuma (Shlach 29 (15 in Warsaw edition), cited by Gra OC 9:3): Nowadays, we have only Lavan, for Techeles was Nignaz (hidden).

Yeshu'os Malko (3): The Ari Zal explains that it was Nignaz because it is to recall all the Mitzvos, and after the Churban we cannot fulfill all of them.

Poskim

Shulchan Aruch (OC 9:2): Nowadays that there is no Techeles, we do not put wool strings on a linen garment or vice-versa.

Aruch ha'Shulchan (2): Years ago, someone claimed that he found the Chilazon and made Techeiles. He attracted some followers; the Gedolim and Klal Yisrael did not agree. It is Batel until Mashi'ach will come.

Teshuvos v'Hanhagos (4:5): Piskei Tosfos (Menachos 118) says that one who deviates is called a commoner, for he makes it as if there are two Toros in Yisrael. Perhaps he transgresses Lo Sisgodedu (making factions). Also, some say that one is Yotzei only if one or two complete strings are Techeiles. If one received this custom from his fathers or Rebbi he may persist, but he must stipulate that if it is not the Mitzvah, he does not intend for the Mitzvah, lest he transgress "Bal Tosif." Others should not begin to wear Techeiles.

Ohr l'Tziyon (2:44:13): If one's fathers did not wear Techeiles, he should not. If it is not really Techeiles, there is concern for Kala Ilan (pretending to fulfill the Mitzvah).