Poryphyry is a deep purple dye made from the marine snail Haustellum brandaris. Rare and expensive, since ancient Roman times, it has been the colour of Emperors, and restricted to their use alone. Varying shades of purple can be created by watering down the dye, and an entire heirarchy of shadings developed in Constantiople to depict social standing. However, if an over-ambitious courtier should happen to wear a shade too close to the Imperial poryphyry, punishments ranged from exile to death.
Poryphyry was the only source of purple dye until the nineteenth century, when synthetic dyes were developed. Therefore, the custom of royalty wearing poryphyry was adopted by many other European peoples as a mark of royalty in imitiation of the Roman practice.