This exercise is geared to allow students to gain basic experiences by completing an simple experiment and using that as a base to design their own experiments to answer their own questions. My students and I have used salt to make basic crystals and we started out trying to find out if impurities in a crystal can change its color. We already know that we can have many different crystals with essentially the same chemical composition showing different colors.
Numerous examples abound in the hall.
The most serious problem dealt with viewing these small crystals. The dissecting microscopes pictured above is a great instrument for this purpose. Regular scopes can also be used but can get messy while hand lens are my third choice. Hand lens allows students to view basic color and shape but do not allow students to view internal structure. The pictures on this page were taken using a microscope.