foliation: from the same root that refers to leaves of a tree or leaves in a book, refers to the tendency of minerals in metamorphic rocks to become oriented and stacked like pages in a book.

Slate:  Microscopic platy minerals are oriented causing the rock to break into flat sheets.

Schist:  larger platy minerals (often mica) are oriented parallel to each other as shown at right, making the rock foliated and often giving it a shiny appearance.
 
 

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Gneiss:  Larger scale organization of minerals produces alternating dark and light bands in the rock.
 

Gneiss:

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