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.
Even today, a deep purple hue remains a promient symbol of royal status; one curious modern example is the tradtion of the engineering students at Queens University in Kingston, Ontario, Canada. For more than 90 years, the engineering students have dyed their gold leather jackets a deep purple upon completing their first year of study; this practice started at the behest of Queen Mary of England in 1913, who wished to honour the engineers of the RMS Titanic, who went down with their ship in her famous sinking in 1912.