As we walk along the trail, some of the colors we see in the rocks and minerals have a different origin than our previous chapter. While transition metals are one of the most common sources of color in minerals, another mechanism is called a color center. It’s also sometimes called an F-center, where the F stands for farbe which is German for color. The ultimate source for the color comes from high energy radiation that interacts with the mineral or crystal. The radiation could come in many forms, including high energy x-rays, gamma rays, or ultraviolet rays which we discussed when introducing electromagnetic radiation way back in Chaps.  1 and 2 . Here on the surface of the Earth we are constantly being bombarded with radiation coming from the cosmos.