This is a dark gaiwan of Yixing clay, glazed with white color on the inside. A gaiwan is a type of Chinese tea cup with lid, and is a popular way to prepare tea in China. You use the lid as a strainer and move the tea, put on the lid, and by a smaller gap, pour out the tea.
Gaiwan is often used as a teapot to pour into a pitcher, but you can also drink directly from a gaiwan like a teacup, where the lid is used to move the tea aside.
Dimensions: The gaiwan is about 8 x 8 cm high with lid and foot, about 5 x 8 cm the cup itself. Suitable for approx. 1 dl. Filled with water right up to the edge, it holds 1.5 dl.