The color purple’s ties to kings and queens date back to ancient world, where it was prized for its bold hues and often reserved for the upper crust. The Persian king Cyrus adopted a purple tunic as his royal uniform, and some Roman emperors forbid their citizens from wearing purple clothing under penalty of death.

Purple was especially revered in the Byzantine Empire. Its rulers wore flowing purple robes and signed their edicts in purple ink, and their children were described as being “born in the purple.”

The reason for purple’s regal reputation comes down to a simple case of supply and demand. For centuries, the purple dye trade was centered in the ancient Phoenician city of Tyre in modern day Lebanon. The Phoenicians’ “Tyrian purple” came from a species of sea snail now known as Bolinus brandaris, and it was so exceedingly rare that it became worth its weight in gold. To harvest it, dye-makers had to crack open the snail’s shell, extract a purple-producing mucus and expose it to sunlight for a precise amount of time. It took as many as 250,000 mollusks to yield just one ounce of usable dye, but the result was a vibrant and long-lasting shade of purple.

Clothes made from the dye were exorbitantly expensive—a pound of purple wool cost more than most people earned in a year—so they naturally became the calling card of the rich and powerful. It also didn’t hurt that Tyrian purple was said to resemble the color of clotted blood—a shade that supposedly carried divine connotations.

The royal class’ purple monopoly finally waned after the fall of the Byzantine empire in the 15th century, but the color didn’t become more widely available until the 1850s, when the first synthetic dyes hit the market.