A gemstone is a precious or semi-precious stone, which is used for making jewelry and is used as an ornament due to its antiquity. Gemmology is the branch of mineralogy, which deals with the study of natural and artificial gemstones. The Earth's crust is composed of three types of rocks, including metamorphic, igneous and sedimentary. Each type of rock originates under various conditions resulting in a diverse range of gemstones. They can be categorized as precious gemstones and semi-precious gemstones. Precious gemstones include sapphires, diamonds, emeralds and rubies. All other gemstones are classified as semi-precious gemstones. Geological conditions like redox conditions, temperature and chemistry determine the amount of trace chemical components. The difference between a gemstone and mineral specimens can be determined through such trace chemical components. The color of the gemstone is determined by these components and can provide a basis for identifying the gemstone’s origin. This book provides comprehensive insights on gemmology. Researchers and students in this field will be assisted by it.