Common name of the vat dye
whose scientific name is 6,6'-dibromoindigotin (C15
Chemically analogous to the blue dye indigo, Tyrian Purple was one of the few reliable purple dyes in existence before mauve was synthesised by Perkin in the late Victorian period. It was, however, very expensive, as it could only be extracted in minute quantities from the shells of the tiny sea-snails of the genus Murex.
Historically, the centre for the production of the dye was the Lebanese city of Tyre, where the snails were found in large quantities. Even in ancient times, however, the dye was so expensive that only the very rich could afford it, and by the later years of the Roman Empire its use was restricted to the Emperors.
Nowadays, the dye can be synthesized by a process similar to that used commercially to make the indigo dye used to dye jeans. It has fallen out of use, however, as there are synthetic purples available that are both cheaper and more reliable.
An interesting thing to note is that the solution of the dye used in the dyeing process is actually transparent and colourless, the purple colour develops gradually upon exposure to oxygen and UV light.