Gold, according to Roman sources. And the Roman government hated this fact.

The Chinese--and the numerous middlemen along the silk road--weren't particularly interested in Roman exports, so often the only way to get Chinese silks were by paying for them with hard currency. This made for a trade imbalance, wherein the Chinese (and again, numerous middlemen) were sitting on piles of Roman gold and all the Romans got in return were rich people in fancy clothes.

Pliny the Elder:

"... at the lowest computation, India and Seres [China] and the [Arabian] Peninsula together drain our empire of one hundred million sesterces every year. That is the price that our luxuries [silk, spices] and our womankind cost us."

Archaeology has disproved this somewhat, as the work of Roman artisans has been discovered in China. Glassware, pottery, etc. However, I think it's safe to say there was a trade imbalance. The Romans just didn't have anything that was a) hugely in demand b) unavailable in China and c) light and resilient enough to make the trip.

The issue got bad enough that the Senate tried forbid men to wear silk. The ban didn't work.