Tekhelet is the Biblical Hebrew word for a certain shade of blue. In the book of Numbers (15:37-39) we read,
“G-d spoke to Moses saying: Speak to the children of Israel and say to them that they should make for themselves Tzitzit on the corners of their garments throughout their generations. And they shall put on each corner a thread of Tekhelet…And you shall see it and remember all the commandments of G-d and you shall do them”
This is the source for the wearing of fringes on garments (or the Tallit of today). In addition, we learn that on each fringe should be an extra thread of Tekheelet. The matter of this Tekhelet thread is a controversial one today with different streams in Judaism expressing different views as to the obligation to wear this thread. Why?
It is written in Bamidbar Raba 17:5 that we only have white strings today as the Tekhelet has been hidden. We know that in ancient times blue dies we derived from snails and were considered highly precious to the extent that the wearing of blue became associated with royalty. In the times of the Romans purple and blue dyes came under imperial control and this drove Jewish Tekhelet production underground. When the Arabs conquered Israel in 683 CE the Tekhelet process was lost altogether.
Some individuals over the centuries such as the Radzyner Rebbe and Rabbi Isaac Herzog took an interest in the mystery of the origin of Tekhelet. Their contributions together with ongoing research concluded that the Murex trunculus, a Mediterranean mollusk, is the source of the Tekhelet dye.
There are constant developments in the field of the search for Tekhelet and therefore many Rabbis today will not advocate wearing Tekhelet. However, other Rabbis are of the opinion that there is absolutely nothing to lose by wearing the Tekhelet that we have today that may possibly be the genuine Tekhelet dye. It is preferable to ask one’s personal Rabbi what his opinion of Tekhelet is and it is worthwhile checking out this fascinating website for a fuller picture of this intriguing commandment http://www.tekhelet.com/.
There are also different opinions as how to tie the Tekhelet on one’s Tzitzit. The Vilna Gaon, Rambam, Sefer HaChinuch and Chabad, to name just a few, all have their own methods. The question of how to tie your Tekhelet if you have decided to take the commandment upon yourself is also worth asking a Halachic authority.