For centuries, dyed fabrics ranked among the most expensive objects of the ancient Mediterranean world, fetching up to 20 times their weight in gold. Huge fortunes were made from and lost to them, and battles were fought over control of the industry. The few who knew the dyes’ complex secrets carefully guarded the valuable knowledge. The Rarest Blue tells the amazing story of tekhelet, or hyacinth blue, the elusive sky-blue dye mentioned 50 times in the Hebrew Bible. The Minoans discovered it; the Phoenicians stole the technique; 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 with the fall of the Roman Empire, the technique was lost to the ages. Then, in the nineteenth century, a marine biologist saw a fisherman smearing his shirt with snail guts, marveling as the yellow stains turned sky blue. But what was the secret? At the same time, a Hasidic master obsessed with reviving the ancient tradition posited that the source wasn’t a snail at all but a squid. Bitter fighting ensued until another rabbi discovered that one of them was wrong—but had an unscrupulous chemist deliberately deceived him? Baruch Sterman brilliantly recounts the complete, amazing story of this sacred dye that changed the color of history.