Korean pottery, which nowadays attracts the highest praise from international collectors, is typically divided into three groups: Cheongja (blue-green celadon), Buncheong (slip-coated stoneware), and Baekja (white porcelain). Celadon refers to Korean stoneware which underwent major development in the hands of Goryeo potters some 700 to 1,000 years ago. Celadon pottery is marked by an attractive jade blue surface and the unique Korean inlay technique used to decorate it. Gangjin of Jeollanam-do and Buan of Jeollabuk-do were its two main producers during the Goryeo Period (918-1392). White porcelain ware represents the ceramic art of the Joseon Period (1392-1910). While some of these porcelain wares display a milky white surface, many are decorated with a great variety of designs painted in oxidized iron, copper, or the priceless cobalt blue pigment imported from Persia via China. The Royal Court of Joseon ran its own kilns in Gwangju, Gyeonggi-do, producing products of the very highest quality. The advanced techniques used in the production of white porcelain wares were introduced to Japan by Joseon potters kidnapped during the Imjin Waeran (Japanese Invasion of Korea, 1592-1598).