Much of the attraction, value and romance of gems lies in their beauty, rarity, and exotic origins. With ever increasing advances in techniques for gem synthesis and for improving the appearance of natural gems, traditional perceptions of gem rarity and uniqueness are changing. Using primarily ruby and emerald as examples, this lecture examines such questions as: Why are natural gems rare? What are the geologic processes that form them? Where are they found? Are natural gems different from man-made gems? What is done to improve the appearance of natural gems? Are such treatments permanent and do they affect gem values? Answers to these and other questions provides perspective on some of the many conflicting claims and confusion surrounding natural and man-made gems, and provide a stepping stone for understanding the unique processes and circumstances responsible for gem formation.