Techelet is the sky-blue dye colour that was familiar in ancient Israel, best known from the Shema’s command to “put upon the fringe of each corner a thread of blue”.

For centuries the practice of putting a blue thread on the corners of the tzitzit lapsed. Under the Romans, blue and purple signified imperial power. Wearing those colours was restricted to Roman elites and the Jewish techelet industry was apparently suppressed. The midrashic Bamidbar Rabbah (17:5) put it: “Now we have only white, for the original techelet has been hidden.”

Placing techelet on the tzitizit again was a long-held Jewish dream. It was considered to have great spiritual power. Rabbi Meir said: “Whoever observes the mitzvah of tzitzit, is considered as if he greeted the Divine Presence, for techelet resembles the sea, and the sea resembles the sky, and the sky resembles God’s holy throne.”

The fascinating story of techelet’s recovery began in 1858 when a French zoologist found that three Mediterranean molluscs produced purple-blue dyes. He determined one of them, murex trunculus, to be the source of the ancient biblical blue. In 1913 the Chief Rabbi of Ireland, Rabbi Isaac Herzog, wrote a dissertation endorsing the claims about murex trunculus and discrediting the case for a squid made by the Radziner Chasidim. Today in Israel, many religious Jews here once again wear a thread of blue on each corner of the tzitzit.

Tallit sh’kula techelet (a tallit that is all blue) is a colloquial expression meaning something overly perfect — derived from the cloak that Korah wore, according to the Midrash, as a challenge to the leadership of Moses.