The "Techelet" Dye - תכלת
Please note that within this document, the term "Tzitzit" refers to the fringe-strings which are worn on the corners of four-cornered garments.
The Dye Itself
Description of the "Chilazon" creature, from which the dye is taken: Menachot 44a
The Chilazon, from which the Techelet for Tzitzit is extracted, was found in Zevulun's portion in Israel: Megillah 6a
The color of techelet: R. Berachot 9b "Techelet"
How the dye is processed: Menachot 42b
The dye's color, as reminiscent of the sea, which reminds of the heavens, which reminds of the Divine Throne: Menachot 43b
Authenticity
How to test the dye, to ensure its authenticity: Menachot 42b-43a
Invalidation of the dye if it used in a test of the batch's authenticity: Menachot 42b
Invalidation of the dye, if mixed with some of the dye which was used for testing: Menachot 40a, 42b
It is impossible to test the dye to see whether some of it has been used in an earlier test of authenticity: Menachot 42b-43a
One may only purchase Techelet from an expert: Menachot 42b
Concern that the public may accidentally use an incorrect dye entitled "Kele Ilan": Menachot 40a
Assuming authenticity of Techelet which was purchased from a Jew, from a Nochri merchant, or from a Nochri non-merchant: Menachot 43a
Whether one may use dyed thread he finds on the street, without knowing that the thread was dyed for the sake of the Mitzvah: Eruvin 96b
Hashem warns us: I knew whether an Egyptian child was a firstborn, I will also be able to tell if you lend money on interest through a third-party nochri, if you store your weights in salt, and if you use indigo instead of techelet on your tzitzit: Bava Metzia 61b
Extraneous Issues
Using its Color as a Determination of Daytime, for recitation of the Morning "Shema": Berachot 9b, Tos. Berachot 8b #4; Menachot 43b
Merchants of Techelet will not profit, unless their motive is altruistic: Pesachim 50b
Whether Merchants of Techelet will profit, if they are careful to check the authenticity of their dye with both of the established methods: Menachot 43a
It is worse to not have the white strings than to not have the Techeletd strings, because the white strings are easier to get: Menachot 43b
Use of the Dye
Whether Tzitzit are valid without Techelet: Menachot 38a-b
Using Tzitzit which, between the garment and strings, have a mixture of wool and linen, if one doesn't have Techelet: Menachot 40a