What is Silk?
People make silk from the cocoons of silkworms. Have you ever seen a butterfly cocoon? It is just like that. You have to take zillions of these cocoons and unwind them carefully, and that makes long threads like spider webs, and you spin these together to make them thicker, and then weave them to make silk.
Silkworms will only eat fresh mulberry leaves, and for a long time mulberry trees only grew in China and Japan (and East Asia generally), and so all silk that people had in India or West Asia or Africa or in the Mediterranean or Europe had to be brought from East Asia.
Traders probably began to bring silk from East Asia to India and West Asia around 2000 BC. By the time of the Roman and Parthian Empires, silk was very popular in West Asia and around the Mediterranean and a lot of silk was being imported. Because it had to come from so far away, silk was very very expensive. Ordinary people could not afford to wear silk. But everyone wanted to wear silk. It was very pretty, smooth and shiny and soft, and comfortable to wear. Also it was cooler in the summertime than wool or linen.
Here's a video of women in China unwinding the cocoons
Around 600 AD, some Christian monks who had gone to China managed to smuggle out two baby mulberry trees and some silkworms under their tunics, and brought them back to West Asia. Soon these silkworms were making silk in Syria, and silk became a lot cheaper than it had been before.
When the Islamic Empire took over Syria less than a hundred years later, it also took over the silk business. Because of this, silk was generally much cheaper and more available in the Islamic Empire than it was in medieval Europe.
Silk, by Claire Llewellyn (2002). For kids.
Eyewitness: Costume, by L. Rowland-Warne (2000). For kids, but mainly European clothing, from earliest times to modern.
World Textiles: A Concise History, by Mary Schoeser (2003). For adults.
Chinese Silk: A Cultural History, by Shelagh Vainker (2004).