Demantoid Garnet Deposits: Review and Geological Classification

The aim of this extended review and update is to present the state-of-the-art on the geology and gemology of the demantoid variety of andradite garnet. A number of studies conducted over the last thirty years have provided the opportunity to unravel the features of the gem andradite and its deposits through field and laboratory research, including the history of their discovery, their geology, geodynamic framework, petrography and mineralogy, solid and fluid inclusions, chemistry, and stable oxygen isotopes. The different features are compared in order to propose a geological classification and a genetic model for the formation of these deposits and finally to approach the question on the geological and geographical origins of the demantoid and why it is so rare on Earth.