Why specifically the chilazon?

Question

The Torah commands us in Bamidbar 15:38 to attach tzitziyot on the four corners of our garments, and insert a thread of blue in each tzitzit.

Speak to the children of Israel and you shall say to them that they shall make for themselves fringes on the corners of their garments, throughout their generations, and they shall affix a thread of sky blue [wool] on the fringe of each corner.

(Translation via Chabad.org)

The Gemara (Menachot 42b) informs us, however, that the blue thread cannot be dyed with just anything. We must dye the thread with the blood of a certain sea-creature called the chilazon.

Why must we dye the thread using the blood of the chilazon? Why does the Gemara raise such a big stink about people using kala ilan, indigo, to dye the threads instead of the blood of the chilazon? The color was exactly the same.

Answer 1

The dye extracted from the Chilazon or Murex Trunculus was very expensive and was considered in ancient times to be a luxury. One of the reasons this dye was so expensive, because "twelve thousand snails of Murex brandaris yield no more than 1.4 g of pure dye, enough to colour only the trim of a single garment." [see link - en.wikipedia.org/wiki/Tyrian_purple]. The process of obtaining the dye and colouring the garments was also time and money consuming, thus making it one of the most valued items in ancient times. This dye was considered royal and was mainly used by the nobility who were able to afford it. This is also evident from Exodus 28 where some of the priestly garments were to be made from Techelet. But why is the poor Israelite obligated to obtain such an expensive item for his garments?

If we suppose that the reason the Torah commands the Benei Yisrael to attach such an expensive string to their garments is - it should serve them as a constant reminder that they are a nation worthy of royalty ("Yisroel Benei Meluchim Heim". Tractate Shabbat 67a) a "chosen nation" a distinguished race, then we can easily understand the disgrace of one who wears "Kla Ilan" instead of "Techelet". That is, besides for the fact that he's misguiding everyone into thinking that he's wearing the expensive Techelet (fooling everyone that he can afford it, or that he is a god fearing Jew), he is also deliberately corrupting the intent of this Mitzvah, and is exchanging the high quality of Techelet for a cheaper quality blue, whereas the color of the Techelet was never the purpose of this commandment (even though the Gemara mentions that the color resembles the color of the Godly throne, that is a "remez bealma") but the quality of this dye!

Answer 2

"Techelet is wool dyed with Chilazon" - this seems to be a "well know" tradition/fact/definition as we see in Menachot 42b:

אמר ליה אביי לרב שמואל בר רב יהודה הא תכילתא היכי צבעיתו לה אמר ליה מייתינן דם חלזון וסמנין ורמינן להו ביורה... ‏

Abaye asks Rav Shmuel the son of Rav Yehuda: "that Techelet, how do you color it?" He answered: "you take the blood of a Chilazon and chemicals and you boil it in a vat..."

The Gemara doesn't even bring a "source" (e.g. a Tosefta, Braitha or the like) for this "fact".

And that's how Rashi defines Techelet in Shemoth 25:4

ותכלת. צמר צבוע בדם חלזון (מנחות מד.), וצבעו ירוק‏

If you use Kala Ilan it isn't Techelet; it's [dark-]blue wool.

So why Techelet and not any [dark-]blue wool that would remind you of the sea-sky-fear of Gcd?

Rashi answers this in Bemidbar 15:41:

פתיל תכלת. על שם שכול בכורות. תרגומו של שכול תכלא.‏
ומכתם היתה בלילה וכן צבע התכלת דומה לצבע רקיע המשחיר לעת ערב.‏
ושמונה חוטים שבה, כנגד שמונה ימים ששהו ישראל משיצאו ממצרים עד שאמרו שירה על הים:‏

Techelet shares the root letters - T.Ch.L - with Tichleh the Aramaic word for killing (the Egyptian firstborns).
They were killed at night and Techelet has the color of the sky as night approaches.
And 8 threads like the 8 days from the time they left Egypt till they crossed they said Shira at the sea.

^{See the Sifsei Chachamim for how to explain the "8 days". Essentially it starts the count from the time they slaughtered the Pessach, on Nissan 14.}