Also to consider is Fluorescence. This effect causes the stone to react to ultra-violet light, e.g. sunlight. In medium to strong presence it will cause the stone to look cloudy or milky and drastically reduce its lustre. I do not generally use a diamond with fluorescence unless required to. In the photo below the two diamonds are exposed to ultraviolet light. While this is a concentrated form, you can see the dramatic effect on the stone with fluorescence.