Refinement in all the arts characterized the Goryeo period, whose potters achieved a pinnacle of Korean ceramic production with elegant gray-green glazed stoneware, known as celadon. Korean potters exceeded mere emulation of Chinese models to develop vessel shapes and decorative techniques that would express their own aesthetics. The Korean invention of inlaid decoration (sanggam) using white and black slips is perhaps the most dramatic of these innovations. Goryeo celadons are admired for aesthetic restraint, a simplicity that belies the technical sophistication required for their creation, and harmony between decorated and undecorated areas.