Clay is characterized as:

1) Fine-grained earth materials  formed by the decomposition of igneous rock.

When combined with water, clay is plastic enough to be shaped; when dry, it is strong; when subjected to red heat or above, it will become progressively more dense and rock-like; and
2) A compound of decomposed and altered feldspathic rock consisting of various hydrated silicates of aluminum along with non-plastics, such as quartz, and organic matter. It is also used as a source of alumina and silica in glazes.