Pearls are a vast subject worthy of years of study. There are nacreous and non-nacreous pearls, each with subcategories of varieties. Prices, colors, qualities, sizes are seemingly endless and it takes great effort to learn about all the possibilities. Here, we take a closer look at the categories. When planning this article, I decided the sensible place to start would be a simple concise definition of pearls and explore from there. The dictionary definitions were simple, the Oxford English dictionary defines a pearl as, “A hard, lustrous spherical mass, typically white or bluish-grey, formed within the shell of a pearl oyster or other bivalve mollusk and highly prized as a gem.” The American Merriam-Webster defines pearl as, “A dense variously colored and usually lustrous concretion formed of concentric layers of mother-of-pearl as an abnormal growth within the shell of some mollusks and used as a gem.” The concise gemological definition was much harder, neither Gem-A nor GIA provided me with a snappy one-liner, and looking to CIBJO I got reams of intertwined and cross referenced definitions. It seems in gemological terms that there is no “simple” definition, and scrutinizing the dictionary definitions they do not align with the CIBJO definitions of pearls entirely. So, what is a pearl? After much reading and unravelling of the CIBJO tome, I tried to distil the many paragraphs down to what I felt was a sensible and simple definition, and came up with this. A pearl is a concentric formation of conchoilin and/or calcium carbonate produced by a mollusk. Within this simple definition lies a myriad of possibilities. The Oxford English dictionary is wrong on a few counts; pearls...