Tekhelet is an azure-sapphire-midnight blue, fade-resistant dye mentioned approximately 49 times in the Tanakh, believed to be God’s chosen color for ancient Israel and a prized shade of blue within the Jewish faith. This specific hue of blue is referenced in the clothing and headdresses worn by Israel’s High Priest and the tapestries of the Tabernacle. The hue has become largely symbolic of nobility, which is consistent with the purpose of the tallit—reminding a Jewish person of his special relationship with God and God’s commandments.

The Lord said to Moses, "Speak to the Israelites and say to them: ‘Throughout the generations to come, you are to make tassels on the corners of your garments, with a blue cord on each tassel. You will have these tassels to look at, and so you will remember all the commands of the Lord, that you may obey them and not prostitute yourselves by chasing after the lusts of your own hearts and eyes. Then you will remember to obey all my commands and will be consecrated to your God. I am the Lord your God, who brought you out of Egypt to be your God. I am the Lord your God.’" Numbers 15:37-42

He made the curtain of blue, purple, and crimson yarn and fine linen, with cherubim worked into it. 2 Chronicles 3:14

Finely embroidered linen from Egypt was your sail and served as your banner; your awnings were blue and purple from the coasts of Elishah. Ezekiel 27:7

While tekhelet blue textiles were once more widely traded commodities throughout ancient times, tekhelet became a luxury item exclusive to royalty and divinity after Jerusalem fell to the Romans and was found in the occasional tzitzit. The dye is sourced from marine life, though the exact source of marine life has been debated over the centuries. The Talmud cites tekhelet as being derived exclusively from the hillazon (alternative spelling, chilazon, believed to be a Mediterranean Sea shellfish creature), while other scholars and rabbis have tried to extract tekhelet dye from God’s other marine creations over time.

Later, tekhelet became nearly impossible to produce during the Middle Ages, with textile dye industry workers forgetting not only where to source the dye or having trouble physically sourcing the dye even if they did know the source but also the exact method of manufacture over the years. The Bible does not specify either a source or a method of production for tekhelet dye. Because of its scarcity, the dye was likely more expensive, likely rendering it less affordable for the average person.

The dye was prized for its fade-resistant beauty even in ancient times because hillazon were not always abundant, and some Talmudic writings suggest the not-clearly-identified sea creature only emerged 70 years before being impossible to find whatsoever. Thus, because the creature was so hard to isolate, the tekhelet dye in its pure form was in scarce supply.

Other rabbis and scholars have tried to manufacture the dye from cuttlefish or by extracting the secretions of a certain marine snail species and exposing the secretions to oxidation via ultraviolet radiation to generate the desired blue dye, but without the same results as the elusive hillazon. At best, the results may have been a sky-blue dye rather than the azure blue of ancient times, leading to debates over what the exact hue of blue should be that rabbis and scholars were attempting to recreate. Was it a sapphire blue or more of a violet blue, based on some wool excavated in Masada? That became controversial. Maimonides believed tekhelet to be more associated with a sky blue or light blue, or, as he described it, "a clear noonday sky."

Modern marine biologists and religious leaders believe the dye originates from a snail species located in the Mediterranean Sea near Haifa. Some rabbis believe the hillazon and its tekhelet dye in their purest, most unadulterated form will not return until the coming of the Messiah.

As a result of the unknown source and controversy over the tekhelet blue dye, many Jewish people have worn white tzitzit over the centuries. The blue that is found on tallits today is believed to be symbolic of the ancient tekhelet dye, but it is not the actual dye in and of itself. This same tekhelet blue has also been incorporated into many Chanukah celebrations.

The significance of the ancient tekhelet dye and its use in the early tallits also factored into the incorporation of the blue stripes and blue Star of David on the modern Israeli flag.

"Indeed, we have a flag, white and blue," said David Wolffsohn, designer of Israel’s flag as proposed to the First Zionist Congress in 1897. "This tallit in which we wrap ourselves in prayer—this tallit is our emblem."

"The decline of the tekhelet dye industry [and along with it, the skill required to produce these dyes] after the Muslim conquest in the 7th century CE led to the disappearance of the blue thread from the tzitzit," said Yehuda Kaplan, a curator at the tekhelet exhibit at the Bible Lands Museum. "Now not only an expression of Jewish personal devotion that survived millennia of historical transformations but also imprinted as a modern symbol of national pride and determination on a global stage, tekhelet blue lives on."