Walking along the trail, the rocks and minerals all around bring color into the world. Where I’m at the colors are fairly subtle, with shades are brown, red, and sometimes green. But we know rocks and minerals can have vibrant colors which is why they are often featured in jewelry. Recall that many of our previous color mechanisms were related to electronic transitions. For sodium, it was a very simple process whereby an electron in an excited state drops down into the ground state and then emits a photon. For fluorescence, it’s a similar process except the electron in an excited state spends some time losing energy through non-radiative processes before eventually emitting a photon of lower energy. In this chapter, we’ll discuss a very common color mechanism which is unique to transition metals. In fact, a large fraction of the color we see in the mineral world is due to the presence of transition metals either as the main component of the material or even as an impurity.