Also known as: Pegmatite. Graven. Gowen. Manx stone. Meldon stone. Growan. Petuntse. Amakusa.

Cornwall Stone is a not a single material, but a mixture mined from igneous rock, making it a mix of different components including feldspar, quartz, kaolinite, mica, and a small amount of feldspar. Because of the material's kaolinite content, it responds well to flocculation and deflocculation in glaze slurries and will help glazes adhere to bisque, prior to firing. 

Cornwall Stone is primarily used as a flux in clay bodies and glazes. It is often used as a substitute for feldspar because it can introduce different alkalis into the glaze like potash, calcia, magnesia, and soda. 

Information gathered from Digitalfire Ceramic Materials Database and The Potter's Dictionary of Materials and Techniques by Frank Hamer.