This is a vintage Japanese Sokogawara (底瓦) used in traditional tea ceremony. This is glazed in a rare turquoise Oribe-ware style with chrysanthemums. 

A sokogawara is a flat ceramic tile traditionally placed under a kama (iron kettle) in a Japanese tea ceremony, particularly in the ro (sunken hearth) setting. Its main purpose is to protect the hearth floor from intense heat and to provide a stable base for the kettle. Made from heat-resistant clay, it also contributes to the rustic aesthetic valued in wabi-cha style tea gatherings.