Tekhelet (Hebrew: תכלת təḵêleṯ, "blue-violet", or "blue", or "turquoise" (alternate spellings include tekheleth, t'chelet, techelet and techeiles) is a blue dye mentioned 49 times in the Hebrew Bible/Tanakh. It was used in the clothing of the High Priest, the tapestries in the Tabernacle, and the tassels (Hebrew: ציצית, Tzitzit (or Ṣiṣiyot) [tsiˈtsit], pl. Tzitziyot or Ṣiṣiyot) affixed to the corners of one's four-cornered garment, such as the Tallit (garment worn during prayer, usually).
In the Septuagint, tekhelet was translated into Greek as hyakinthos (ὑακίνθος, "hyacinth").
According to the Talmud, the dye of Tekhelet was produced from a marine creature known as the Ḥillazon (also spelled Chilazon). According to the Tosefta (Men. 9:6), the Ḥillazon is the exclusive source of the dye.
After the destruction of the Temple in Jerusalem by the Romans, the sole use of the Tekhelet dye was in Tzitzit. A set of Tzitzit consists of four tassels, some of their strands being Tekhelet, which Rashi describes as green as “poireau,” the French word for leek, transliterated into Hebrew. There are three opinions in Rabbinic literature as to how many are to be blue: 2 strings; 1 string; 1 half string. These strands are then threaded and hang down, appearing to be eight. The four strands are passed through a hole 25 to 50 mm away from the corners of the four-cornered cloth.
Tekhelet is mentioned in the third paragraph of the daily prayers known as the Sh'ma Yisrael (Hebrew: שְׁמַע יִשְׂרָאֵל; "Hear, [O] Israel"), citing Bemidbar – Parashat Shelakh (Book of Numbers 15:37–41).
At some point following the Roman destruction of the Second Temple, the actual identity of the source of the dye was lost, and during a period of over 1,400 years, most Jews have only worn plain white tassles (Tzitzit).
The stripes on prayer shawls, often black, but also blue or purple, are believed by many to symbolize what the majority opinion in mainstream Judaism considers as the lost Tekhelet which is referred to by various sources as being "black as midnight", "blue as the midday sky", and even purple. These stripes of tekhelet inspired the design of the flag of Israel.
Various sea creatures have been suggested as the Ḥillazon, the purported source of the blue dye. None have been universally accepted, though the murex, Hexaplex trunculus, commonly referred to in literature by the now defunct name Murex trunculus, is currently thought in mainstream Judaism to be the most likely source of the biblical blue dye.
In 1887, Grand Rabbi Gershon Henoch Leiner, the Radziner Rebbe, researched the subject and concluded that Sepia officinalis (common cuttlefish) met many of the criteria. Within a year, Radziner chassidim began wearing Tzitzit that included threads dyed with a colorant produced from this cephalopod. Some Breslov Hasidim also adopted this custom due to Rebbi Nachman of Breslov's pronouncement on the great importance of wearing Tekhelet and in emulation of Rabbi Avraham ben Nachram of Tulchyn, a prominent Breslov teacher who accepted the view of his contemporary, the Radziner Rebbe.
Rabbi Yitzhak HaLevi Herzog (1889–1959) obtained a sample of this dye and had it chemically analyzed. The chemists concluded that it was a well-known synthetic dye "Prussian blue" made by reacting Iron(II) sulfate with an organic material. In this case, the cuttlefish only supplied the organic material which could have as easily been supplied from a vast array of organic sources (e.g., ox blood). R. Herzog thus rejected the cuttlefish as the Ḥillazon and some suggest that had the Grand Rabbi Gershon Henoch Leiner known this fact, he too would have rejected it based on his explicit criterion that the blue color must come from the animal and that all other additives are permitted solely to aid the color in adhering to the wool.
Within his doctoral research on the subject of Tekhelet, Herzog placed great hopes on demonstrating that Hexaplex trunculus was the genuine snail Ḥillazon. However, having failed to consistently achieve blue dye from Hexaplex trunculus, he wrote: “If for the present all hope is to be abandoned of rediscovering the Ḥillazon Shel Tekhelet in some species of the genera Murex (now "Hexaplex") and Purpura we could do worse than suggest Janthina as a not improbable identification”. Although blue dye has in the meantime been obtained from Hexaplex trunculus snail, in 2002 Dr. S. W. Kaplan of Rehovot, Israel, sought to investigate Herzog's suggestion that Tekhelet came from the extract of Janthina. After fifteen years of research he concluded that Janthina was not the ancient source of the blue dye.
In his doctoral thesis (London, 1913) on the subject, Rabbi Herzog named Hexaplex trunculus (then known by the name "Murex trunculus") as the most likely candidate for the dye's source. Though Hexaplex trunculus fulfilled many of the Talmudic criteria, Rabbi Herzog's inability to consistently obtain blue dye (sometimes the dye was purple) from the snail precluded him from declaring it to be the dye source.
According to Zvi Koren, a professor of chemistry, Tekhelet was close in color to midnight blue. This conclusion was reached based on the chemical analysis of a 2000-year old patch of dyed fabric recovered from Masada in the 1960s. The sample, shown to have been dyed with Murex snail extraction, is a midnight blue with a purplish hue. Additionally, in 2013, Na'ama Sukenik of the Israel Antiquities Authority verified a 1st-century CE-dated fragment of blue-dyed fabric to have used H. trunculus as the source of its pure blue color.
In the 1980s, Otto Elsner, a chemist from the Shenkar College of Fibers in Israel, discovered that if a solution of the dye was exposed to ultraviolet rays, such as from sunlight, blue instead of purple was consistently produced. In 1988, Rabbi Eliyahu Tavger dyed Tekhelet from H. trunculus for the Mitzvah (commandment) of Tzitzit for the first time in recent history. Based on this work, four years later, the Ptil Tekhelet Organization was founded to educate about the dye production process and to make the dye available for all who desire to use it. The television show The Naked Archaeologist interviews an Israeli scientist who also makes the claim that this mollusk is the correct animal. A demonstration of the production of the blue dye using sunlight to produce the blue color is shown. The dye is extracted from the hypobranchial gland of Hexaplex trunculus snails.
In conclusion, its color varies between central sky blue in sunlight, and the iris indigo of the rainbow; translated in a German translation of the biblical book of Exodus, as "blauem purpur".
Tekhelet is ancient Hebrew for blue-violet, used in Modern Hebrew with the meaning of "light blue". Karaite Jews believe that the importance of Tekhelet is that the color of thread is blue rather than it being necessarily a specific dye. Additionally, it is also believed by the Karaites that the Rabbinic tradition that the dye is produced from a mollusc known in Rabbinic tradition as the "Ḥillazon" is incorrect, because such an impure (a definition mostly overlapping "un-Kasher", or "treif") source would be prohibited by the Torah, proposing instead that the source of the dye was indigo or woad (the "Asp of Jerusalem" plant Isatis tinctoria, used as a fast dye in Ancient Egypt).