physics-fan3.14, posted: "For those reading this thread who are unfamiliar with this term, please read the brief summary on wikipedia here: https://en.wikipedia.org/wiki/Chatoyancy A tiger's eye is one of the best examples of chatoyancy in gems. The key quote from the summary on wikipedia is this: The difference between the gem and a coin is that the fibres in a gem which reflect the light are all arranged in a parallel fashion. The ridges that produce cartwheel luster are arranged radially around the coin, converging on the center (usually, depending on the design). Thus, when you rotate the gem, you don't see a cartwheel - you see the chatoyancy as unidirectional. However, when you rotate the coin, you are changing which set of parallel lines you are reflecting light off of, according to which direction your light is angled. The principle which is creating the effect, that of light bouncing off microscopic disturbances, however, is exactly the same. A wheel mark, or hairlines from polishing, are absolutely going to be chatoyant as you mention. However, their stark unidirectional reflective pattern is the fundamental difference between that and natural luster - they don't have that cartwheel, and just as the gem, may be nearly invisible at the wrong angles. "Natural mint luster" should be radial, not unidirectional. Based on this explanation, and my extrapolation of the term to numismatic, I stand by my assertion that this word may have some application in this discussion." That's why I wrote: "Hold the coin rock steady to mimic chatoyancy with a coin." Then, the radials going from the center to 1 o'clock will line up with the radials going from the center to 7 o'clock. They line up in a basically parallel direction! It mimics the gem. I hope the term never gets into the numismatic lexicon.