Apparatus for generating data for determining a property of a gemstone and methods for determining a property of a gemstone

An apparatus for generating image data for use in determining a visual property of a gemstone [20] perceptible to the human eye, such as a cut diamond, the apparatus comprising: a support structure for supporting a gemstone placed an observation position, the support structure being capable of supporting a gemstone having an axis of symmetry so that the axis of symmetry is parallel to an X [22] axis passing through the observation position; an illumination means [24, 26, 28] arranged to illuminate a gemstone placed that way with a light pattern spatially varied; rotation means arranged to cause relative rotation between the light pattern and the support structure generally about the X [22] axis; and a camera [30] arranged to capture electronic images of light returned by the gemstone and to output said images as image data; characterized in that: the aparatocomprende control means [36] arranged to control the rotation means and camera [30] such that the camera [30] captures an electronic image of the gemstone during said relative rotation at each of the plurality of different rotational positions of the support structure relative to the light pattern, the images being captured generally along the axis X [22].