Stoneware is a kind of pottery — cups, bowls, plates, jars, teapots — made from a clay that is fired very hot, so hot that the clay itself turns hard and glassy inside and no longer soaks up water. That is the whole point of the word: it is heavier and tougher than everyday earthenware, and unlike fine porcelain you cannot see light through it. It usually feels thick and a little rough, and the colour tends to be grey, brown, or a warm off-white. Because it is strong and holds heat, stoneware is what people choose for casserole dishes, mugs, and pots for pickling or storing food. The word is uncountable — you say 'a set of stoneware' or 'a stoneware bowl', never 'two stonewares'. Using it in front of another noun like that ('a stoneware mug') is very common. It is a plain, ordinary word in shops and cooking, not a technical term, though potters use it precisely to name one of the three main clay types. Note it has nothing to do with stone: it is clay, and 'stone' here only describes how hard and stony the finished thing feels.