How to classify rocks.
The classification of rocks is based on two criteria, texture and composition. The texture has to do with the sizes and shapes of mineral grains and other constituents in a rock, and how these sizes and shapes relate to each other. Such factors are controlled by the process which formed the rock. Because igneous, sedimentary, and metamorphic processes are distinct, so too the resulting textures are distinct. Thus there are distinct igneous textures, distinct sedimentary texture, and distinct metamorphic textures. For the purposes of this exercise and routine classification, the kinds of minerals and their proportions, or mineralogy, are taken as the natural expression of composition. Fortunately for you, just as the three classes of rocks each have distinct textures, so too do they have distinct mineralogies. Details of texture and composition are discussed in the individual sections on igneous, sedimentary and metamorphic rocks.
Here is a link that you can check out about classifying rocks. http://www.appstate.edu/~abbottrn/rck-id/
Here is a link that you can check out about classifying rocks. http://www.appstate.edu/~abbottrn/rck-id/