Tzaydei hillazon – The fishers of the hillazon are from Haifa to Sulamot Shel Tzur (the Ladders of Tyre). (Shabbat 26a).
Archeological digs show remnants of the dyeing industry on the Northern coast of Israel through the southern coast of Lebanon. (Royal Purple, p.149-157).
Digs near Haifa and Tyre and beyond, revealed mounds of Murex shells (broken to access their dyestuff) – some up to one hundred yards long and several yards thick. (Royal Purple, p.24, p.151-5; Ziderman, p.438; Twerski, p.82).
Potzeia – One who breaks open a hillazon … (Shabbat 75a). Go and learn [about the clothes of the Jews in the desert] from the hillazon, all the time that it grows, its shell (nartiko) grows with it (Shir HaShirim R. 4:11).
R. Herzog explains the use of the verb potzea to mean, “break open” – as in a nut. (Herzog, p.57).
The Murex snail is a hard-shelled Mollusk, which must be broken open to obtain the dyestuff. (Royal Purple, p.180; Ziderman, p.430).
The shells found in the archeological digs were broken in the exact spot necessary to obtain the dyestuff. (Ziderman, p.438).
“The hillazon is this: its body is like the sea, it’s creation is like a fish, it comes up once in 70 years and with its blood one dyes tekhelet – due to this it is expensive” (Men. 44a).
The vagueness of these descriptions make them ineffective for use in identification – other more indicative signs could have been given, if that was the intention of the Gemara. Each point comes to explain the conclusion of the statement that “the dye is expensive” (Rock, n.57).
The declaration that “it is expensive” is simply out of place in a formal halachic definition. It would, however, make sense as part of an explanation to consumers curious as to the reason for the exorbitant price. (Herzog, pp.66-7).