Abstract: The method of determining the authenticity and the geographical origin of gemstones of crystalline structure comprises the steps consisting in: applying an electromagnetic beam to the gemstone; determining values associated with the absorbance of the gemstone for wavelengths of the beam in an absorption direction that is predetermined relative to a characteristic axis of the crystal; calculating at least one ratio between these values; and comparing the or each ratio with predetermined corresponding ratios belonging to gemstones of predetermined authenticity and origin.