This is a sake cup called "Gyokuhai," which has a color and texture similar to jade (hisui). Because natural stone has high thermal conductivity, it keeps cold sake from warming up quickly, and it is said to make the taste of the sake even smoother with its cool touch. In addition to being used as a sake cup, it can also be displayed as a small item holder or accessory tray. I bought one for 10,000 yen.