Silk was the most luxurious fabric available to medieval Europeans, and it was so costly that only the upper classes -- and the Church -- could attain it. While its beauty made it a highly-prized status symbol, silk has practical aspects that made it much sought-after (then and now): it's lightweight yet strong, resists soil, has excellent dyeing properties and is cool and comfortable in warmer weather.
The Lucrative Secret of Silk
For millennia, the secret of how silk was made was jealously guarded by the Chinese. Silk was an important part of China's economy; entire villages would engage in the production of silk, or sericulture, and they could live off the profits of their labors for much of the year. Some of the luxurious fabric they produced would find its way along the Silk Road to Europe, where only the wealthiest could afford it.
Eventually, the secret leaked out of China. By the second century C.E., silk was being produced in India, and a few centuries later, in Japan. By the fifth century, silk production had found its way to the middle east. Still, it remained a mystery in the west, where craftsmen learned to dye it and weave it, but still didn't know how to make it. By the sixth century, the demand for silk was so strong in the Byzantine Empire that the emperor, Justinian, decided they should be privy to the secret, as well.
According to Procopius, Justinian questioned a pair of monks from India who claimed to know the secret of sericulture. They promised the emperor they could acquire silk for him without having to procure it from the Persians, with whom the Byzantines were at war. When pressed, they at last shared the secret of how silk was made: it was spun by worms.1 Moreover, these worms fed primarily on the leaves of the mulberry tree. The worms themselves could not be transported away from India . . . but their eggs could be.
As unlikely as the monks' explanation may have sounded, Justinian was willing to take a chance. He sponsored them on a return trip to India with the objective of bringing back silkworm eggs. This they did by hiding the eggs in the hollow centers of their bamboo canes. The silkworms born from these eggs were the progenitors of all the silkworms used to produce silk in the west for the next 1,300 years.
Medieval European Silk Producers
Thanks to Justinian's wily monk friends, Byzantines were the first to establish a silk production industry in the medieval west, and they maintained a monopoly on it for several hundred years. They set up silk factories, which were known as "gynaecea" because the workers were all women. Like serfs, silk workers were bound to these factories by law and could not leave to work or live elsewhere without the permission of the owners.
Western Europeans imported silks from Byzantium, but they continued to import them from India and the Far East, as well. Wherever it came from, the fabric was so costly that its use was reserved for church ceremony and cathedral decorations.
The Byzantine monopoly was broken when Muslims, who had conquered Persia and acquired the secret of silk, brought the knowledge to Sicily and Spain; from there, it spread to Italy. In these European regions, workshops were established by local rulers, who retained control over the lucrative industry. Like the gynaecea, they employed mainly women who were bound to the workshops. By the 13th century European silk was competing successfully with Byzantine products. For most of the Middle Ages, silk production spread no further in Europe, until a few factories were set up in France in the 15th century.
1The silkworm isn't really a worm but the pupa of the Bombyx mori moth.