General Knowledge
Why was purple dye once worth more than gold?
- It took thousands of sea snails
- The plants were rare
- It was banned
The answer is It took thousands of sea snails. Tyrian purple came from murex snails - thousands per gram.
Tyrian purple came from a gland in murex sea snails, and the arithmetic was brutal - something like ten thousand snails for a single gram of dye. The mounds of discarded shells outside the Phoenician cities of Tyre and Sidon were large enough to be landmarks. Weight for weight it cost more than gold, and Diocletian's price edict of AD 301 lists purple-dyed wool at figures that read like a misprint.
It became a legal monopoly because it was already an economic one. Roman sumptuary laws restricted who could wear it until, in the later empire, a fully purple garment was reserved for the emperor and putting one on amounted to a declaration of treason. The Byzantine phrase born in the purple comes from the porphyry-lined chamber where a reigning emperor's children were delivered.
The chemistry supplies the punchline. Tyrian purple is 6,6'-dibromoindigo - indigo, the dye in a pair of jeans, with two bromine atoms attached. The most exclusive substance in the ancient world and the most common one in the modern are close chemical relatives. Production collapsed with Constantinople in 1453 and the technique had to be reconstructed from scratch centuries later.