Numbers 15 commands a thread of tekhelet, a particular blue, among the fringes. For most of the last fifteen centuries no Jew has worn one, because nobody could say with confidence what tekhelet was.
The Talmud says the dye comes from a creature called the hillazon, and gives fragmentary indications: it comes up from the sea at intervals, its body resembles the sea, its form resembles a fish, and the dye it yields is indistinguishable by eye from a vegetable imitation called kala ilan, which is why a test was needed. The identification was lost after the Roman and Byzantine periods, along with the industry, and rather than risk using a wrong dye the practice simply stopped. White fringes are the result.
Three attempts to recover it are worth recording. In the 1880s the Radzyner Rebbe, Gershon Henoch Leiner, published a study and produced a dye from cuttlefish, which his followers wear to this day; chemical analysis later indicated that the blue in his process came from the added iron rather than from the animal, which most authorities took as disqualifying. In the 1910s Isaac Herzog, later chief rabbi of Israel, argued in a doctoral thesis for a sea snail, Murex trunculus, but was troubled that the dye it produced came out purple rather than blue. That objection was answered decades later when it was shown that exposing the dye solution to sunlight during the process shifts the result to blue, and that ancient dyers could readily have done so.
Many now wear Murex tekhelet. Many communities do not, on the ground that a chain of tradition once broken cannot be restored by argument, however good. The identification is a reconstruction, and it is held as one.