Key Minor Facets Affecting Scintillation

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To get the most out of this article, you will need to familiarise yourself with the following:

Optical Cut Analysis - We will be using lightscope images extensively
Names of Diamond Facets - We need to explain how the facets interact with light

Do not worry yourself if you cannot remember these. Just refer to them whenever you need to.

Pattern & Scintillation

Scintillation is the movement of light around the diamond caused by the movement of the light source, the movement of the diamond, the movement of the observer or a combination of the above.

If you have gone through the article on Optical Cut Analysis, you will understand that blacks on the lightscope images represent light reflected directly from above the diamond. Reds represent light reflected from above the diamond at an angle, and whites are light from the bottom of the diamond. To achieve good scintillation, we need the following:

1. The diamond must have good light return, represented by black and red areas (white shows light leakage).
2. A good mix of blacks and reds (AGS sets the benchmark at 18% black).
3. The blacks must be well dispersed and not clumped together.

Obviously, if you have a good representation of black and red areas that are well distributed, the light reflected from the diamond will keep changing angles as the light moves, thus creating more scintillation.

As such, the pattern of the diamond is important. Take a look at the 2 lightscope images (taken with GoodOldGold's DiamXray) below:

It is quite clear that the diamond on the right will have better scintillation.

Both diamonds are cut to Tolkowsky's ideal proportions with good symmetry and polish, and both offer good light return. How then, are the patterns of light return so different for these 2 diamonds? This is what we will explain here.

Lower Girdle Facets (or Pavilion Girdle Facets)

The lengths of the lower girdle facets are important for a few reasons.

1. It deterimines how thick or thin the arrows are.
2. It determines the appearance of various hotspots.

Measuring the Length of the Lower Girdle Facets

The most convenient way to measure the length of the lower girdle facets is to use one of many available machines.

When measuring lower girdle facet length using the Sarin hardware/software solution, the edge of the girdle to the culet is the 100% reference point as measured along the horizontal plane (indicated by the red arrow in the picture on the right.

Lower girdles are measured on the horizontal plane, so when we examine the graphic from it's profile view we clearly see how this metric is determined (indicated by the green arrows).

Some machines (like the OGI, OctoNus Helium or OctoNus Diamcalc) take the measure from the vertical plane (taking the depth of the lower girdle facet vs the depth of the pavilion) as per the picture on the left. Although the concept is the same, the results of the lower girdle facet length measured by these 2 methods are different.

[Click here for the conversion chart from Lower Girdle Length to Lower Girdle Depth]

I shall use the Lower Girdle Length measure for our analysis because it is the same one used by GIA's cut grading. However, you may use Lower Girdle Depth if you prefer.

Thick or Thin Arrows?

Arrows are simply light internally reflected off the pavilion mains (highlighted in yellow). The arrow tips are the pavilion mains as seen though the bezel facets. The arrow shanks are the pavilion mains as seen through the table.

Red = Lower Girdle Facets | Yellow = Pavilion Mains

If we lengthen the lower girdle facets (highlighted in red), the pavilion mains will have to be thinner has the lower girdle facets compete for space. Thus the thinner arrows.

More or Less Hotspots?

There are essentially 2 types of hotspots. The first type are the triangular hotspots like these the ones pointed out by the blue arrows. We shall call these 'hotspots at the stars'. The second type are the small hotspots pointed out by the green arrows. We shall call these 'hotspots at the arrow shafts'.

Hotspot at the stars (triangles pointed by blue arrows) are caused by the internal reflection of light off the lower girdle facets as seen through the star facets. So, if we lengthen the lower girdle facets, these hotspots will also appear longer. Note that since these hotspots appear through the star facets, lengthening the star facets will also help to enlarge them (which we will discuss later).

Hotspots at the arrow shafts (small hotspots pointed out by green arrows) are caused by the internal reflection of the lower girdle facets as seen through the centre of the table. These appear around the arrow shafts. When the lower girdle facets are long enough, these will appear.

Case Study

We inlcude a few simulated lightscope images to illustrate how the length of the lower girdle facets will affect the optical quality of a diamond. Note that the star facet lengths are kept constant at 55%, to better demonstrate the effect of the lower girdle facets.

Average Lower Girdle Lengths*	Simulated LightScope Image	Comments
71% Lower Girdle		Short lower girdles equals fat arrows. Those wide pavilion main facets do provide for strong fire but reduced scintillation. There are no hotspots at the stars and none at the arrow shafts. It is generally agreed that there is a little too much black caused by the the pavilion mains (arrows).
76% Lower Girdle		Increasing the lower girdle facet length to around 76% produces better light return through the stars. Hotspots at the stars are small, but present. The arrows are better proportioned within the diamond and a better sense of "balance" is acheived. 76% lower girdles enter a zone that is more visually appealing.
79% Lower Girdle		Lengthening the lower girdles to 79% gives more pronounced scintillation through the stars. We can now see the presence of more blacks at the arrow shafts (see the green arrows).  These diamonds are some of the most aesthetically beautiful stones within the H&A diamonds and are usually TEAMING with fire and scintillation.
82% Lower Girdle		We can lengthen the lower girdles further to 82%. This will increase the size of the hotspots at both the stars and the arrow shafts.

* Note that Star Lengths are kept constant at 55%.

As you can see, the optical effect of a diamond responds significantly to changes in the length of the lower girdle facet. Therefore, it is too important to ignore.

Star Facets

Star facets are named as such because they form a star on the crown of a diamond as per the picture on the right. Since it is the exact area where the hotpsots at the stars appear, changing its size will affect the appearance of the hotspots. That is why we are especially interested in these facets.

Measuring the Length of Star Facets

Like the lower girdle facets, the most convenient way to measure the length of the Star facets is to use one of the many available scanning machines.

When measuring the star facet length using the Sarin hardware/software solution, the edge of the girdle to the edge of the table is the 100% reference point as measured along the horizontal plane (indicated by the red arrow).

Star lengths are measured on the horizontal plane, so when we examine the graphic from it's profile view we clearly see how this metric is determined (indicated by the green arrow).

Case Study

We inlcude a few simulated lightscope images to illustrate how the length of the star facets will affect the look of a diamond. Note that the lower girdle facet lengths are kept constant at 78%, to demonstrate the effect of the star facets.

Average Star Facet Lengths*	Simulated LightScope Image	Comments
43% Stars		Rich dark blacks can exist in short stars only if the lower girdles are cut to proper length. Since the lower girdle length is 78%, we see a healthy spread of blacks even when the star facet lenght is only 43%. However, the blacks can be improved.
55% Stars		Blacks under the stars are stretched slightly enhancing fire at those points and increasing the scintillation of the diamond.
65% Stars		Blacks are lengthened further. 65% stars creates very nice hotspots. However, we do not want to lengthen the star facets too much or the diamond will look unbalanced, with very short upper girdle facets. Therefore around 65% stars should be the limit.

Where to find information on Lower Girdle Facet and Star Facet lengths?

You can find information on Lower Girdle Facet and Star Facet Lengths in many reports, such as the following reports:

• GIA FacetWare Report -which uses the Sarin Scan
• Helium Report - Note that OctoNus measures lower girdle facet depth, but they do provide the lower girdle facet length as well. Be careful which one you rely on.

The GIA Diamond Report also contains some information on Lower Girdle Facet and Star Facet Lengths. However, these are rounded to the nearest 5%. So, they are not as granular. The AGS Diamond Quality Document is better in ths aspect because it rounds to the closest 1%.

What are the desired Lower Girdle Facet and Star Facet lengths?

Which facet lengths you finally choose is a matter of taste. However, there are a few things to take note of:

1. Try to pick out diamonds that have lower girdle facet lengths of at least 76%. At 76%, the arrows are thicker. But the hotspots at the stars are still well represented.
2. You can choose higher lower girdle facet lengths if you like thinner arrows and more hotspots.
3. Try to pick out diamonds that have star facet lengths of at least 55%. This will lengthen the hotspots at the stars.

Ultimately, which diamond you pick out is a matter of taste. Now that you are armed with knowledge on these important minor facets, you can select the diamond you desire based on information, rather than chance. With the amount of money you will be spending on a diamond I am sure you would like to know exactly what you are buying. Check out my most recent purchase (which is the diamond featured in the reports above):

Up next, we will discuss other important consequences of poor cut proportions..