The swirled concentric layering of yellow-brown and brown colors is called liesagang. It is a common weathering phenomena. The gray color in the center, where the penny is, is the fresh, unweathered color of the rock. Because rocks are often broken into blocks along joints, ground water percolates along the joints causing weathering to proceed from the outside, inward. During chemical weathering soluble ferrous (Fe+2) iron is released and carried by the water into the pores of the rock opened by weathering. Later the iron precipitates as insoluble ferric iron (Fe+3) forming the liesagang concentric layers. In time even the gray area inside will weather and turn yellow-brown, before the rock disintegrates completely.
Contributed by Lynn Fichter