Stoneware is a type of ceramic made from fire clay at a kiln. Much like green glass (and clear glass, to a lesser extent), stoneware items can be produced in unlimited quantities as long as you have a source of fire clay in your fortress. They are gray in color and have a material value of 4, greater than that of earthenware despite not actually being more difficult to produce.
Unlike earthenware, stoneware containers can be used to store liquids without being glazed.
Stoneware tea bowls, Song Dynasty