Slate is a very fine grained foliated rock composed of tiny mica flakes. Since the mica flakes are so small, slate is not usually visibly foliated. Slate does, though, exhibit cleavage which is evidence that minerals are aligned, therefore, foliated.

Slate is considered low-grade metamorphism of shale, although, it may be a form of metamorphosed volcanic ash as well.

Slate can be almost any color depending on its mineral content. Black slate contains organic material, red slate has iron oxide and green slate usually has chlorite.

Information provided by Patrick M. Colgan, Northeastern University
http://www.casdn.neu.edu