For centuries, dyed fabrics ranked among the most desirable objects of the ancient world, fetching up to 20 times their weight in gold. Few people knew their complex secrets, carefully guarding the valuable knowledge. The Rarest Blue tells the incredible story of tekhelet, or hyacinth blue, the elusive sky-blue dye mentioned throughout the Hebrew Bible. Minoans discovered it; Phoenicians stole it; Roman emperors revered it; Cleopatra adored it; and Jews—obeying a Biblical commandment to affix a single thread of the radiant color to the corner of their garments—risked their lives for it. But as the Roman Empire dissolved, the color vanished. Then, in the nineteenth century, a marine biologist marveled as yellow snail-gut smeared on a fisherman’s shirt turned blue. What had caused this incredible transformation? Meanwhile, a Hasidic master obsessed with the ancient technique posited that the source of the color dye was no snail but a squid. Bitter controversy divided European Jews until a brilliant Rabbi proved one side wrong. But had people been deceived by an unscrupulous chemist? In this richly illustrated book, Baruch Sterman brilliantly recounts the amazing story of this sacred dye that changed the color of history.