You may wish to do granite with the group, modeling the process you go through to determine that the sample is granite. Have the students repeat this process until all of the samples have been identified. Then have students, in groups, answer the associated questions. You may wish to add questions based on your own specific suite of samples. Take a field trip to a local igneous rock outcrop.
Granite quarries are ideal, as are pegmatite locations.
Pay special attention to the changes in texture and composition near the contacts between rock units. Click here for a photo of igneous and metamophic activity. Each tray will hold up to twenty 3x4 inch specimens. You can number the white compartment sections with a grease pencil and make up kits for different labs as needed. Make certain that all specimens get put into the trays in proper sequence.