In essence, clay is a mixture of rocks, chemicals, minerals, and other organic matter. Did I just describe mud in a fancy way? Maybe. But, it’s the truth! Clay can be all of these things, and what differentiates one type from another is the ratio of these components in the clay body.
Porcelain, stoneware, and earthenware are all just mixtures of different basic organic and inorganic compounds, which become pottery at different firing temperatures. If we got any more specific we’d be reading off those chains of random letters with tiny numbers attached called chemical formulas.
Read more about clay, here.