China's import of silver

China's currency in the middle ages was silver, which meant that it was hugely important for the West as a large demand for silver helped the West's silver mines stay alive.

From books.debt (Private):

By 1540, a silver glut caused a collapse in prices across Europe; the American mines would, at this point, simply have stopped functioning and the entire American colonization process foundered, had it not been for demand from China.

By the late 16th century, China was importing almost 50 tons of silver a year, ~90% of its silver, and in the 17th centure, 116 tons, over 97%.