In Numbers 15:38, God commands the Israelites to dye a cord of their tzitzit (tassels) blue: “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.'”
English Bible translations typically translate the Hebrew word tekhelet as “blue.” But recent scholarship—as reported by the New York Times—is bringing attention to the extreme complexity and difficulty of producing the color tekhelet:
[Dr. Koren] found that the dye used in the Masada sample, a piece of bluish-purple yarn embroidery, came from a breed of Murex trunculus snail familiar to modern Israelis. Such shades on textiles are rare finds since they were typically worn exclusively by royalty or nobility.
Determining what exactly tekhelet would have looked like in its day has been the subject of conjecture and curiosity among rabbis, religious commentators and scientists for centuries; it is considered the most important of the three ritual colors cited in the Bible. The other two are argaman, a reddish purple, and shani, known as scarlet.
“It’s especially exciting for religious Jews who place great importance on this color,” said Daniella E. Bar-Yosef Mayer, a University of Haifa archaeologist specializing in mollusk shells.
Some time after the Jews were exiled from Israel in A.D. 70, the knowledge of how to produce the tekhelet dye was lost. The dye was also prohibitively expensive to make: hundreds of snails were used to make even a small batch, and some in ancient times claimed it was worth 20 times its weight in gold.
“Blue” may be the most straightforward English translation of the term, but as is often the case in translating Hebrew, nuances are lost:
“Tekhelet is the color of the sky,” Dr. Koren said in his laboratory. “It’s not the color of the sky as we know it; it’s the color of sky at midnight.” He paused and added, “It’s when you are all alone at night that you reach out to God, and that is what tekhelet reminds you of.”