"Rice grain" is a name of a decorative technique rather than actual rice grains or anything like inserted in the walls. To make this kind of decoration the holes are pierced through the rather thick walls of the rough and unfired porcelain objects and the holes then filled with translucent glaze. When the entire unfired porcelain piece have dried enough to be handled, the walls are thinned down manually until the walls are as thin as the potter dares to make them.
The highly skilled potters, who are usually young women, after what I saw when I visited a factory for this kind of porcelain in Jingdezhen in the 1990s - are judging the thickness of the walls and the work progress by the sound of the paring knife against the unfired clay.
Originally this technique seems to have come to China from Turkey during the 14th century. The "rice grain" technique seems never to entirely have been abandoned even if it at times was rare. Sometimes the openings was left unfilled, which resulted in a wicker work appearance. This open work was called ling long yan (eye-like openwork) or 'devil's work'.
The Chinese scholars have it that this work was pioneered at the Hongzhou kilns around Luohu, Fengcheng, Jiangxi province, durong the Sui and Tang dynasties. In the Yongle reign of the Ming, extremely fine porcelains with the 'ling long' decorations was made in the Imperial kilns. After the Qianlong reign of the Qing dynasty both the Imperial and the folk kilns manufactured such wares.
Openwork porcelain also exists half and fully pierced in export porcelain from the later part of the Ming dynasty.
Today "rice grain" porcelain is common, since it is much appreciated and can be bought over the counter in most Asian food and porcelain stores all over the world. The last years products do not differ much from earlier product beside that they might be made with slightly more machines and that part of their decoration might be stamped, but all in all there are not much there to help you see a difference in the pieces from anywhere during the entire 20th century.