Gems are often common minerals with uncommon aesthetic properties. The related industries (from mining to jewelry) generate billions of US dollars every year throughout the world, and employ a wide variety of crafts. The understanding of gem formation and possible subsequent modifications therefore is of enormous economic interest. Gemstones occur in various geological environments, from mantle (e.g. diamond) to upper lithosphere (e.g. opal). The study of geochemistry of gem materials (trace elements, isotope composition), their petrology (solid and fluid inclusions, paragenesis) and spectroscopic data are used to define the geological conditions of gem genesis and, hence, to unravel the specific conditions that make these minerals unusually attractive. In turn, studying gemstones helps highlighting the genesis of their "common" counterparts, because gems usually are remarkably free of fractures, alteration and other post-growth features. Stimulated by the fact that gem materials are valuable, they are commonly treated to enhance their visual appearance. The synthesis techniques that are used commonly to make gems available at lower price also provide valuable information on natural gems’ growth conditions. However, treatment and synthesis are very much an issue in the gem industry. We welcome contributions on all gems, their genesis, treatments and synthesis.