Silk was itself a primary currency along the Silk Roads, and was used as currency within China for much of its history, since it was not only a desired good, but much lighter and easier to transport than metal currency. From the Western ends of the Silk Roads gold, silver, copper, and bronze coins were used to purchase the goods transported from the Middle East, Central Asia, and East Asia. Some trade was supported by gems as currency from India and Central Asia. (Whitfield, Susan. Life Along the Silk Road. 21.)

Silk was used by China for the salaries paid to soldiers stationed along the Chinese portions of the Silk Roads as early as the Han, with the expectation that they would use the silk in trade to provide for themselves. (Liu, Xinru. The Silk Roads: A Brief History with Documents. 6.)

The Kushan, Sogdian, and Sassanid coinage were used and honored or exchangeable along much of the Silk Roads at various times, here I will recommend reading Valerie Hansen’s The Silk Road: A New History, as she goes into the archaeological details much more than this post can. I would also recommend the entirety of Xinru Liu’s book above, as it is a very useful collection of translated documents from all along the Silk Roads, as well as her The Silk Road in World History.