An apparatus for generating image data for use in determining a visual property of a gemstone perceptible to the human eye, such as a cut diamond, the apparatus comprising: a support structure for supporting a gemstone placed at an observation position, the support structure being capable of supporting a gemstone having an axis of symmetry such that the axis of symmetry of the gemstone is parallel to an axis passing through the observation position; illumination means (24, 26, 28) arranged to illuminate a gemstone supported on said support structure with a spatially varied light pattern; rotation means arranged to cause relative rotation between the light pattern and the support structure generally about said axis; and a camera arranged to capture electronic images of light returned by the gemstone and to output said images as image data; control means arranged to control the rotation means and camera such that the camera captures an electronic image of the gemstone during said relative rotation at each of a plurality of different rotational positions of the support structure relative to the light pattern, the images being captured generally along said axis; the illumination means (24, 26, 28) comprising a reflector having a concave surface arranged to reflect light generally towards the gemstone, wherein the concave surface has a plurality of relatively reflective regions and a plurality of relatively unreflective regions alternately arranged radially about said axis, thereby creating said light pattern, the apparatus characterised in that: the boundaries between adjacent relatively reflective and unreflective regions having lengths greater than the length of a notional straight line along the concave surface from the centre of the concave surface to the closest point of the edge of the concave surface. a support structure for supporting a gemstone placed at an observation position, the support structure being capable of supporting a gemstone having an axis of symmetry such that the axis of symmetry of the gemstone is parallel to an axis passing through the observation position; illumination means (24, 26, 28) arranged to illuminate a gemstone supported on said support structure with a spatially varied light pattern; rotation means arranged to cause relative rotation between the light pattern and the support structure generally about said axis; and a camera arranged to capture electronic images of light returned by the gemstone and to output said images as image data; control means arranged to control the rotation means and camera such that the camera captures an electronic image of the gemstone during said relative rotation at each of a plurality of different rotational positions of the support structure relative to the light pattern, the images being captured generally along said axis; the illumination means (24, 26, 28) comprising a reflector having a concave surface arranged to reflect light generally towards the gemstone, wherein the concave surface has a plurality of relatively reflective regions and a plurality of relatively unreflective regions alternately arranged radially about said axis, thereby creating said light pattern, the apparatus characterised in that: the boundaries between adjacent relatively reflective and unreflective regions having lengths greater than the length of a notional straight line along the concave surface from the centre of the concave surface to the closest point of the edge of the concave surface. the boundaries between adjacent relatively reflective and unreflective regions having lengths greater than the length of a notional straight line along the concave surface from the centre of the concave surface to the closest point of the edge of the concave surface.