Assuming that the grading lab is both qualified and competent in establishing these points of comparison and maintains a strong ethical position with regard to “bias” toward the accuracy of their grading, one can still get a false sense of understanding the actual differences between stones which can only be determined by using your own eyes to form an opinion or make a comparison. 

The fact of the matter is that many stones just don’t look as good as their report might lead you to expect. Other stones may actually present themselves better in person than their report might lead you to believe. If we are truly trying to understand beauty, Then we must not only see with our own eyes, but also filter that observation through our own set of preferences and reach a conclusion that is personal to each of us.

Some examples follow which may help you to understand:

Diamonds, when not colorless, may exhibit a tint of either yellow, brown, or gray. While two stones may have the same color grade, the subtle difference in body color when faced up may cause one stone to appear whiter or brighter than the other. A diamond that is slightly brown may face up better than a diamond of the same grade that is slightly yellow. Color grading is based primarily on tone (from light to dark) rather then actual color (hue).

Sometimes the presence of faint blue fluorescence may affect the face up color of a diamond in a positive way, even making it appear one or more grades higher.

Always remember that grades are ranges. A stone at the top of the range may have a much better appearance than a stone at the bottom.

A diamond with an inclusion which reflects around the stone in numerous facets will affect the face up appearance in a negative manner. This would not be explained by the actual clarity grade or the plotting of inclusions. 

In fancy shapes, the reflection of an inclusion within the point or corner or in an opposite point or corner may leave areas of the stone less brilliant.

A bow tie effect (the darkening of a central area ) in a marquise, pear, oval, and even a princess cut will not be understood by analyzing a grading report. This occurs because of a number of interrelated aspects of the stone : the length to width ratio, depth and table percentages, culet size, and the unique aspects of the faceting of the stone all play a role in the shadowing or dark reflection through the center of many fancy shaped diamonds.

Windowing (the lack of reflection due to improper facet angles), and the subsequent “see thru” of certain areas of fancy shaped diamonds, in particular, cannot be estimated without direct and careful observation.

The actual shape, symmetry or outline of a fancy shaped diamond may be more or less desirable depending on the viewer’s personal preference. A flattened heel or a pointed lobe will yield a very different looking stone than the mere length and width ratios will define.

While this list is in no way exhaustive, it should serve to clarify to prospective buyers that a grading report is an effective tool in establishing an independent opinion of a diamond. It is also extremely valuable as a preview to actually viewing a stone. It may tell you some of the important points which establish value in a diamond, but it is not always effective in telling you how beautiful a given stone will be. The true value of many diamonds can not be assumed from their reports alone.