In the olden days, you could signify your wealth by wearing gold bracelets, earrings and jewels; but if you wanted to make your opulence clear to everyone, you wore purple. Specifically, you would wear Tyrian Purple, a color that was produced in a single city in the Roman Empire for somewhat unknown reasons. Said to be discovered by Hercules as he wandered along the beach with a Nymph named Tyrus. His dog came bounding up to him with a sea snail in its mouth and when it ate the snail, his dog’s mouth became a deep purple color. The nymph told Hercules that she would marry him if he made a robe the color of the snail goo, and he named it after her (in actuality, it was named after Tyre, the city that produced it).
While dyes in general were difficult to produce and expensive to obtain, Tyrian Purple was the highest bar attainable in the ancient world of colors. When fresh out of the dyeing vat it was described as the color of “coagulated blood, but when held to the light it showed a crimson hue”. What made it so sought after was that wear over time in the sun and elements would cause the color to gradually become purple, becoming brighter and richer as it wore. While most dyes would slowly turn brown or beige, Tyrian Purple would only become more beautiful and brilliant as it was used.
What made it so expensive was that it was only obtainable through a specific mixture of secretions from two varieties of predatory sea snails that were found in the harbor region of Tyre. Buccinum secretions would cause a reddish color that would fade quickly, while Murex secretions would give it a deep purple shade. Nowadays, Murex shellfish are found at 25 fathoms, and rapidly becoming harder to find; but when the Romans used them they were closer to the shore and more populous. 200 pounds of concentrated Buccinum dye would sell for the same amount that 111 pounds of Murex. The Murex dye was obtained by fishing out the mollusk at a specific time of the year around wintertime because the secretion is a bromine solution that the snails produce as an anti-microbial shield on their eggs. The shellfish would secrete the liquid as a defense against predators, but it only produced a useful amount of it “after the rising of the dog star”. Catching the snails around their mating season would allow the Romans to “milk” them. The excretion of the snails could be obtained by poking them until they used it as a defense mechanism, or by crushing the snail and forcibly removing the vein that held the dye.
As for why it was such an expensive dye, Roman scholars said that 1,200 snails would only produce 1.4 grams of the dye when fully concentrated, barely enough dye to color the hem of a robe (each snail had only a few partial ounces of the fluid, and 8,000 pounds of the pulped snail glands would result in only 500 pounds of dye). The snail goo would be collected in a vat and boiled for 10 days to remove all impurities and concentrate the fluid. Because of this long boiling session where snail corpses were burned to a crisp, Tyre was known as an terribly smelly city, “unpleasant for residence”. The end result of the boil was a tiny amount of the radically expensive dye that would be used to make clothes for the richest in Rome and Byzantium. Tyre was one of the transit points of the famous silk road; Silk from the Orient would be woven in Lebanon and later brought to Tyre to be dyed. Because of the wealth that the dye trade brought to it, Tyre was said to be one of the largest commercial cities of its time, beautiful and richly decorated (and stinky). Sadly, with the sacking of Constantinople during the Crusades, the crippled Byzantine empire was no longer economically able to continue producing the dye and ceased. Europe shifted to using Vermilion as the go-to color of wealth and power. Vermilion was produced by crushing insects from the Kermes genus, and was more readily accessible to Western Europe. For that reason, the classic image of a King’s outermost robe being deep red exists: the Europeans destroyed the producers of all things purple. While this is only a guess, it seems plausible that Cardinals in the Catholic Church wear red robes for the same reason, the color of power shifted away from purple, and ecclesiastical princes required a powerful and royal hue (Bishops will still commonly wear purple though, for what it’s worth).
Mosaic of Christ clad in Tyrian Purple in St.Apollinare, Ravenna Italy (http://iconreader.files.wordpress.com/2011/06/tyrian-purple-byz-ravenna6c.jpg)