They can choose any object-as long as it's easy to carry. Have students brush any dirt from their specimen and look at it closely. They can also wash it and look at it with a magnifying glass. Encourage students to observe carefully and form logical hypotheses, rather than try to identify all of the specimens. Knowing a rock or mineral's natural setting can help identify it.
Color can be a clue to minerals.
Iron, for instance, may give rocks a reddish hue. If smooth, it's more likely to have been rounded by the action of water over long periods of time. A shiny surface may indicate the presence of mica or quartz crystals. If so, it is probably made largely of quartz. The color may change, or what was thought to be rock may prove to be dirt or clay.