Home About Us Articles Gallery News Forum Contact

Clarifying the Case of the 'Nail Head' Diamond

This article resolves differences of opinion concerning issues addressed in my "Journal of Gemmology" article, "Diamond Brilliance" of October, 2000. It clarifies and elaborates on the cause of darkness in 'nail-head' diamonds. In section 5 of "Diamond Brilliance" the cause of the table darkening is explored in the so called 'nail-head' diamond. Discussion was raised as to which diamond pavilion angles should be labeled as nail-heads. This article answers that question by clarifying the circumstances that result in the table darkening in diamonds with pavilion main facets of 43 to 45 and greater.

For those who haven't time to read this article, but want to know how it turns out, here is the highlights summary:

  1. The GIA calls a diamond that exhibits a dark table a 'nail-head'.
  2. In "Diamond Brilliance", Section 5, evidence is presented to show that the table darkening, 'nail-head' appearance is not due to light leakage, but rather to retro-reflection from the observer's direction.
  3. Diamonds having pavilion angles of 44 to 45 (and greater) exhibit the classic nail-head, table darkening at normal face-up viewing distances.
  4. Close-up viewing can also result in the nail-head appearance in a diamond with 43 pavilion main angles such as the 'nail head' diamond described in the GIA study.
  5. Pavilion angles close to the Ideal 40.75 or 41 avoid the nail-head appearance in spite of dark pavilion main reflections typically caused by close-up viewer interference.
  6. Because the undesirable 'nail head' appearance is due to retro reflection from the the viewer's direction, it is observable in lighting environments where little or no light is available in the area above the diamond where the normal observer's head is located. The typical dark table appearance is not seen in a 'nail head' diamond with deep pavilion angles under hemispherical lighting because of the absence of light obstruction by the viewer's physical presence.
  7. As observation distance increases, viewer interference decreases. At a great enough viewing distance the nail-head appearance may totally disappear, (except for the 45 case). For example, the dark table appearance of the two 'nail head' diamonds seen in Figure 9b turns bright in the hemisphere illumination of Figure 9a.
  8. Depending upon the viewing circumstances, diamonds with 43 or greater pavilion angles can exhibit the characteristic table darkening and be labled a nail-head.

For more information on the ideas and concepts presented here check out the new update of the "Diamond Brilliance" article previously published in the GEM A's, "JOURNAL OF GEMMOLOGY", October 2000. "Diamond Brilliance Theories, Measurements and Judgement" This article reports research in the area of diamond brilliance and explains differences in observed diamond beauty between the American Ideal cut and other diamond cuts that deviate significantly from the American Ideal’s crown and pavilion angles. For those noticing the frequent reference to the 'normal' viewing angle. The dual meaning of normal makes it a descriptive word for the face-up view of a gemstone. The mathematical meaning of normal to a surface is the perpendicular direction to the surface. The term normal reminds us that the face-up view of a diamond is a view approximately perpendicular or normal to the surface of the diamond's table. In addition, this most important viewing angle is the usual or normal way a diamond is observed and evaluated.

"Diamond Brilliance" stated (Section 5, p.216): "Perhaps the best case to illustrate the need for incorporating the effect of the viewer's physical presence on brilliance is a diamond with pavilion main facets between 43 and 45. This is known as the 'nail-head' diamond owing to its dark appearance under the table relative to areas outside the table. Assignment 8 of the GIA Diamond Grading Course (1993), states: "If the pavilion is very deep, much of the light is leaking out. Then the table reflection and star facets look almost black, and the stone is called a 'nail-head'".

In "Diamond Brilliance", Section 5, evidence is presented to show that the table darkening 'nail-head' appearance is not due to light leakage, but rather to retro-reflection.

Retro Reflection

Contrary to the usual light leakage explanation for the darkness in a deep cut diamond, a nail-head with pavilion main facets of 45 mirrors light from above through the table in those main facets, and rather than leaking the light, reflects it straight back towards its source. In the optics field this is termed 'retro reflection'. A viewer of such a diamond could observe a mirror image of his/her skin tones in those pavilion main facets. Furthermore, the head obscures any illumination from behind, causing those main facets to darken under the table. The pavilion girdle facets, which are cut between 1 and 2 steeper than the mains, also darken under the table giving the whole table area a darkness relative to areas outside the table.

In a nail-head diamond, as the pavilion angle increases from 41 there is a corresponding increase in the viewing distance at which the table remains dark. Thus, the steeper the pavilion angle the further the observer must be from the diamond to avoid interference. Because the viewing distance that causes the reduction in brilliance varies with the pavilion angle, (and the effective size of the viewer's head including hairdo), there can be disagreement as to what diamond angles and proportions constitute a 'nail-head'.

The nail-head example of Hemphill et al. (1998) had a 43 pavilion that only revealed its nail head appearance under the viewing circumstances of close-up observation. Under those circumstances the leftmost diamond in Figure 9b with an average pavilion angle of approximately 43 had the characteristic nail head appearance.


Figure 9
'Nail heads' on left vs. near 'Ideal' cut, rightmost diamond
(a) in hemisphere lighting created by diffusing two fiber optic light sources (b) in hemisphere lighting partially blocked as in close-up inspection

As the observation distance increases, viewer interference decreases. At a great enough viewing distance the nail head appearance may be absent, as it is in the hemisphere illumination of Figure 9a. (The 45 case is one exception.)

Diamonds in Hemisphere Light with and without Viewer Interference

In the diffuse hemisphere lighting photograph (Figure 9a), all three diamonds have similar even brilliance. The near 'Ideal' cut is slighty more brilliant, because the 'nail-head' diamonds have some dark areas in the outer table region. However, the two 'nail-heads' are very bright in the middle portion of their tables. Contrast this with the dramatic darkening of the whole table and star facet areas of both 'nail-head' diamonds in the Figure 9b photograph. This appearance is consistent with appearance of a 'nail-head' diamond because it has accounted for the viewer blocking light directly over the diamond in the normal viewing position.

This is photographic evidence that the typical 'nail head' appearance in a diamond with deep pavilion angles is not seen in hemisphere lighting. It is observable in lighting environments where little or no light is available in the area above the diamond as in the case of close-up inspection by the 'normal' viewer.

The following comparison example shows how the appearance of a mathematically perfect, 'nail-head' diamond and an Ideal cut changes as the diamond is examined at two viewing distances. Keep in mind that the viewer head interference has more effect on brilliance the closer the diamond is viewed. (Of course, the proximity of the whole viewer affects the diamond's brilliance. The head effect is being simulated due to its relatively larger impact on brilliance.)

The 'nail head' diamond in this comparison was chosen for its high light return when viewed from a sufficient distance where the observer blocks little of the diamond's illumination. Note that "Diamond Brilliance" on page 226 states, "The illumination of the diamond has as much influence on the measure of brilliance as do the diamond's proportions. Illuminating a diamond from enough different angles can cause even the most poorly proportioned diamonds to have high light return."

DiamCalc from Octonus Software was used to model a diamond having an even steeper pavilion angle, 43.5, than the 43 pavilion angle that GIA labeled a nail-head. This diamond has a 25 crown angle along with the GIA article's reference proportions for the other facets and 81% pavilion girdle facets.

Figures 1 and 2 provide a comparison of this diamond next to an Ideal cut example, both in identical lighting.


Figure 1.
43.5, 25 diamond with light blocked within 10.5 of the vertical.

Figure 2.
Ideal cut diamond with same illumination and light interference as Figure 1.

These two images model the appearance of an example of a nail-head and an Ideal cut that differ only in the crown and pavilion angle. The illumination models a viewer at a distance that causes light to be blocked from within 10.5 of the perpendicular or 'normal' to the table. At this viewing distance the nail head example has greater light return but less of the snappy contrast aspect of brilliance present in the Ideal cut.

If the viewer now looks at these same two diamonds in identical lighting from a viewing distance 31% closer, the dramatic table-darkening characteristic of the nail-head becomes apparent, see Figure 3. Comparing the Ideal cut of Figure 2 to Figure 4, it remains unaffected in the inner table reflection area, the area outside the table and between the darkening reflections from its main facets.


Figure 3.
43.5, 25 diamond with light blocked within 15.5 of the vertical.

Figure 4.
Ideal cut diamond with same illumination and light interference as Figure 3.

What has been shown by comparing this deep pavilion angle diamond to the American Ideal cut diamond at these two different viewing distances?

When the deep pavilion angle diamond and the American Ideal are viewed from a sufficient distance the nail head appearance is absent. The deep cut diamond may look brighter while the American Ideal appears sharper in appearance due to greater contrast between its facet's light reflections. However, the close-up observation circumstance of Figures 3 and 4 reveals a flaw in the deep cut diamond's beauty. This flaw is the undesirable table darkening of the 'nail head' diamond brought about by the obstruction of light due to the proximity of the viewer. Under this same close-up viewing circumstance, the American Ideal cut remains sharply brilliant with an even greater aspect of contrast brilliance.

The table darkening in Figure 3 is similar to that seen and photographed in the two 'nail head' diamonds in Figure 9B above from "Diamond Brilliance", page 219. It is important to note that in the actual cutting of a nail-head diamond the symmetry is normally poorer and the table is usually larger than that of the diamond in Figures 1 and 3 to improve weight yield. What is observed in typical nail head diamonds, like the two in Figure 9b, is a general darkening of the table area relative to the rest of the diamond surface. Meanwhile, the Ideal cut of Figure 4 remains brilliant as in Figure 2 in the center and throughout with good contrast between reflections from the main facets and the pavilion girdle facets.

In summary,

  1. The GIA calls a diamond that exhibits a dark table a 'nail- head'.
  2. In "Diamond Brilliance", Section 5, evidence is presented to show that the table darkening. 'nail-head' appearance is not due to light leakage, but rather to retro-reflection from the observer's direction.
  3. Diamonds having pavilion angles of 44 to 45 (and greater) exhibit the classic nail-head, table darkening at normal face-up viewing distances.
  4. Close-up viewing can also result in the nail-head appearance in a diamond with 43 pavilion main angles such as the 'nail head' diamond described in the GIA study.
  5. Pavilion angles close to the Ideal 40.75 or 41 avoid the nail-head appearance in spite of dark pavilion main reflections caused by close-up viewer interference.
  6. Because the undesirable 'nail head' appearance is due to retro reflection from the the viewer's direction, it is observable in lighting environments where little or no light is available in the area above the diamond where the 'normal' observer's head is located. The typical dark table appearance is not seen in a 'nail head' diamond with deep pavilion angles under hemispherical lighting because of the absence of light obstruction by the viewer's physical presence.
  7. As observation distance increases, viewer interference decreases. At a great enough viewing distance the nail-head appearance may totally disappear, (except for the 45 case), The dark table appearance of the two 'nail head' diamonds seen in Figure 9b disappears in the hemisphere illumination of Figure 9a.
  8. Depending upon the viewing circumstances, diamonds with 43 or greater pavilion angles can exhibit the characteristic table darkening and be labled a nail-head.

For more information on the ideas and concepts presented here check out the new update of the "Diamond Brilliance" article previously published in the GEM A's, "JOURNAL OF GEMMOLOGY", October 2000. This article reports research in the area of diamond brilliance and explains differences in observed diamond beauty between the American Ideal cut and other diamond cuts that deviate significantly from the American Ideal's crown and pavilion angles.


Home   •   Top of Page   •   Site Map / Alternate Menu  •   Contact Us
Copyright by ACA Gem Laboratory
Web Designed by Ideal Site Builders