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Clues in the shape, chemical composition, and other features of minerals tell geologists the story of their formation. For example, dendrites trace the movement of water through cracks in rock layers. When dendrites occur in moss agate, they indicate variable cycles of water in the surrounding rock. Since rocks may weather changes in their environment without changing themselves, they can act like time capsules telling the history of our planet. However, using geology to interpret stones does not just reveal their history. With a sound understanding of the forces that shape mineral formation, geologists can find treasure maps in the treasures themselves. Since certain minerals only form in association with others, finding one indicates the presence of the other nearby. A water-worn stone found in a gully somewhere may yield clues that a valuable vein of minerals may await discovery somewhere upstream.