A diamond with positive contrast has an even distribution of bright facets and fewer but well distributed dark facets that alternate as the stone is rocked from side to side. Facets which appear bright act as windows and mirrors to direct your line of sight toward light sources or bright areas in the environment. When your line of sight is directed toward a dark part of the environment, or the light leaks out the pavilion into a dark area behind the stone, that facet will appear dark.

To help understand this let us consider some well known examples of diamonds with negative contrast; the ‘nail-head’ which has a very deep pavilion, the ideally proportioned ‘Tolkowsky’ and finally dark shallow diamonds. All three are shown in Fig 2.

Figure 2. Photos of diamonds with pavilion angles reducing from left to right. Note the differences in contrasting darkness and brightness.(These photo’s are from GIA’s Foundation article)

Consumers are often shown simplified ray path sketches to explain diamond cut quality and why there is a loss of brilliance caused by leakage out the pavilion of a diamond; these explanations are wrong! The main cause of darkness is your head and body obstructing light sources. In the examples photo’s shown the camera lens is the source of darkness in the table of the ‘nailhead’ and most of the darkness in the shallow diamond in Fig 2.

Figure 3. Consumer education from www.adiamondisforever.com/buy/4cs_print.html

Figure 4. Consumer education from www.gia.edu

A ‘nail head’ is a round brilliant with a deep pavilion, near 45° or 50% depth (like the left in Fig2). Seen from the face up direction, your line of view from most facets within the table area is directed back toward your head, as in Fig 5. Because your head blocks most potential lights, the table area of the stone appears dark and dull. The ‘bow-tie’ in some marquise, pear shape and oval diamond is also caused by Head Obstruction of light sources. Peep through a small hole in a sheet of copy paper and the bad ‘bow-tie’ disappears. But the dark star pattern in the middle stone in Fig2. is also caused by observer obscuration; this amount of evenly distributed darkness adds contrast which increases our perception of brilliance.

Shallow diamonds leak less light out the pavilion than so called ‘ideal cuts’. To leak like the De Beers shallow example, the pavilion must be abnormally shallow (near or below 25° or 23% pavilion depth). Shallow diamond darkness is also caused by your line of sight being directed back toward your head (obscuration) as shown in Fig 5. Reverse the ray path and the dark crown facets have a line of sight out the table and back in the direction of your head.

Figure 5 Ray paths in shallow, deep ‘nailhead’ and well proportioned diamonds.