 Tribute was paid to Song China and China exported silk, books, spices, glasses, incense, precious stones and textiles, tea, medicine, and ceramics while Goryeo exported gold, silver, copper, ginseng, porcelain, pine nuts, and hanji paper back to the Chinese.The economy of ancient Gojoseon prospered due to improvements in agricultural technology (as iron tools were introduced from China) coupled with an abundance of natural resources like gold, silver, copper, tin, and zinc during the second half of the first millennium CE.Earlier Korean currencies was based on bartering as a means of exchanging goods and services.