Korea Although Korea borrowed from China key aspects of ceramic technology, which in turn were transmitted to Japan, it developed highly individual stonewares and porcelains admired by its two larger neighbors. High-fired stonewares appeared in Korea around 1000 B.C.E. and are characterized by a fine gray body produced by firing in a reducing atmosphere. Such was the refinement of the body that deliberate glazes remained rare, the potters concentrating instead on elegant forms, many of which are thought to have fulfilled a ritual role. During the tenth century rapid improvements in glaze technology occurred, culminating in the famous "kingfisher" celadons of the Koryo period (918-1392). The great majority of the kilns producing celadons were concentrated in the Cholla provinces in the southwest of the peninsula closest to China, and it is possible that an immigration of potters from China may have fostered the advances in glaze technology. Celadons were used in court and Buddhist circles, while unglazed stonewares continued to be produced for the common people. The Chinese envoy Xu Jing who left a record of his visit to Korea in 1123 could not hide his admiration of Korean celadons, while another Chinese author ranked them "first under Heaven."