What is an optical phenomenon? Plainly put, this term embodies the result of the way light interacts with the crystalline structure, inclusions, and/or internal structure of a gemstone. It is, in essence, the reflection, diffraction, absorption, and/or diffusion of that light. Without even knowing, we encounter these phenomena on a daily basis, whether in opal, moonstone, alexandrite, or cat’s eye chrysoberyl—that said, one should be careful not to confuse optical property, such as dispersion in a diamond, with optical phenomena.