Inkstone

An inkstone is a stone mortar for the grinding and containment of ink.
In addition to stone, inkstones are also manufactured from clay, bronze, iron, and porcelain.
The device evolved from a rubbing tool used for rubbing dyes dating around 6000 to 7000 years ago.
Usage.
Traditional East Asian ink is solidified into inksticks. Usually, some water is applied onto the inkstone (by means of a dropper to control the amount of water) before the bottom end of the inkstick is placed on the grinding surface and then gradually ground to produce the ink. 
More water is gradually added during the grinding process to increase the amount of ink produced, the excess flowing down into the reservoir of the inkstone where it will not evaporate as quickly as on the flat grinding surface, until enough ink has been produced for the purpose in question.
The Chinese grind their ink in a circular motion with the end flat on the surface whilst the Japanese push one edge of the end of the inkstick back and forth. 
Water can be stored in a water-holding cavity on the inkstone itself, as was the case for many Song Dynasty (960–1279) inkstones. The water-holding cavity or water reservoir in time became an ink reservoir on later inkstones. Water was usually kept in a ceramic container and sprinkled on the inkstone. The inkstone, together with the ink brush, inkstick and Xuan paper, are the four writing implements traditionally known as the Four Treasures of the Study.
History.
The inkstone is Chinese in origin and is used in calligraphy and painting. Extant inkstones date from early antiquity in China. The device evolved from a rubbing tool used for rubbing dyes dating around 6000 to 7000 years ago. The earliest excavated inkstone is dated from the 3rd century BC, and was discovered in a tomb located in modern Yunmeng, Hubei. Usage of the inkstone was popularized during the Han Dynasty. Stimulated by the social economy and culture, the demand for inkstones increased during the Tang Dynasty (618–905) and reached its height in the Song Dynasty (960–1279). Song Dynasty inkstones can be of great size and often display a delicacy of carving. Song Dynasty inkstones can also exhibit a roughness in their finishing. Dragon designs of the period often reveal an almost humorous rendition; the dragons often seem to smile. From the subsequent Yuan Dynasty, in contrast, dragons display a ferocious appearance. The Qianlong Emperor had his own imperial collection of inkstones catalogued into a twenty-four chapter compendium entitled "Xiqing yanpu" (Hsi-ch'ing yen-p'u). Many of these inkstones are housed in the National Palace Museum collection in Taipei.
Books and scholarship on the various inkstones of China existed chiefly in Japan, where a long bibliography on the subject exists. Inkstones should be appreciated in the context of the traditional scholar's studio culture and the appreciation of furnishings, antiques, paper, seals and all other associated objects. Members of the Chinese literati, such as the Song Dynasty's Ouyang Xiu, contributed greatly to this new culture.
Four Famous Inkstones.
For serious calligraphers and painters, a good inkstone is as important as the quality of the ink. An inkstone will affect the quality and texture of the ink that is ground upon it. Four kinds of Chinese inkstones are especially noted in inkstone art history and are popularly known as the "Four Famous Inkstones".