Tyrian purple, a natural dye, was so expensive that its use was restricted to royalty. It was discovered about fifteen centuries before the Christian era, and the art of using it did not become lost until the eleventh century after Christ. It was obtained from two genera of one species of shellfish. It was estimated that 8,500 shellfish were required to produce one gramme of dye, so only small amounts could be produced and so the price was very high. Ultimately, in later ages, a restrictive policy of the eastern emperors caused the art to be practised by only a few, and around the start of the twelfth century, when Byzantium was suffering from attacks without and dissensions within, the secret of Tyrian purple was lost. The rediscovery of Tyrian purple was made in England around the year 1683 by a Mr. Cole, of Bristol. Before 1856, the only textile dyes available were those found in nature.