1. A method of constructing a 3D virtual model of a gemstone including the steps of: performing measurements of the gemstone to construct a 3D virtual model of an exterior surface of the gemstone; identifying one or more visible inclusions within an interior volume of the gemstone; for each identified inclusion, performing the steps of: determining a location and 3D shape of the inclusion within the interior volume of the gemstone; capturing at least one image of the inclusion; using the at least one image to determine relevant optical characteristics of the inclusion; and constructing a 3D virtual model of the inclusion, said 3D virtual model of the inclusion including the 3D shape of the inclusion and optical properties of the inclusion based upon said optical characteristics; constructing a 3D virtual model of the gemstone, said 3D virtual model of the gemstone including the 3D virtual model of the exterior surface of the gemstone and the 3D virtual models of the one or more visible inclusions within the interior volume of the gemstone; generating a dataset representing said 3D virtual model of the gemstone; and using said dataset in subsequent computer analysis to provide a user with information relating to a visual characteristic of the gemstone; wherein the step of using the at least one image to determine relevant optical characteristics of the inclusion includes the sub-steps of: using the at least one image to assign an inclusion type to the inclusion; and on the basis of the assigned inclusion type, assigning optical characteristics to the inclusion, said optical characteristics being selected according to the inclusion type and properties of the inclusion observable in the at least one image; wherein the optical characteristics include at least one of reflectivity and transmittance.