Tekhelet

Tekhelet (Hebrew: תְּכֵלֶת‎ təḵēleṯ; also transliterated tekheleth, t'chelet, techelet, and techeiles) is a highly valued blue dye that held great significance in ancient Mediterranean civilizations. In the Hebrew Bible and Jewish tradition, tekhelet is used to color the tzitzit (fringes) attached to the corners of four-cornered garments, including the tallit, and historically in the clothing of the High Priest of Israel and tapestries in the Tabernacle.

The Bible does not specify the source or production method of tekhelet, but rabbinic literature records that it could only be derived from a marine animal known as the ḥillāzon (Hebrew: חלזון). However, the knowledge of tekhelet production was lost during the Middle Ages, leading to the omission of tekhelet from tzitzit. In recent times, many Jews believe that experts have identified the ḥillāzon as the snail Hexaplex trunculus (historically classified as Murex trunculus) and rediscovered the process for manufacturing tekhelet, leading to the revival of tekhelet's use in tzitzit.

A garment with tzitzit consists of four tassels, each containing four strings. There are three differing opinions in rabbinic literature regarding the number of strings in each tassel that should be dyed with tekhelet: two strings, one string, or one-half string.