Ru ware(汝窑) is the rarest of all Chinese major ceramic wares and considered as the treasure of the Chinese traditional ceramic firing process. Appeared in the late Northern Song Dynasty, Ru is ranked as the top one of the five famous kilns in the Song Dynast. Ru was named after the kiln in Ruzhou territory (now Baofeng County area, Ruzhou, Henan province). The Ru kiln is famous for producing the celadon which has sky blue, pea green or powder blue major glaze colors. Because of only twenty years firing time, Ru ware was rare even during the southern Song Dynasty. In the early Ming Dynasty, Jingdezhen imperial ceramic kilns tried to produce the imitation of Ru ware. In addition, Nian Xiyao and Tang Ying also made the imitation of Ru ware during Qing Yongzheng and Qianlong period. Now, only 67 pieces of real Song Ru wares were found in the world.

Ru ware has very fine and smooth porcelain clay, thinner body and thicker glaze. The surface of the Ru ware is covered with reddish-brown crackles caused by the different cools and contracts speed between the glaze and the body. Crackles on Ru wares are considered as a special artistic decoration rather than a defect. The glaze of Ru ware was mixed with the agate powder and other materials which contains iron oxide and turns greenish when fired. The glaze of Ru ware may show azure, green beans, light green and sky blue different colors. The color of Ru ware is usually described as the blue of the sky after raining.