Every waking moment of our lives, as we view our own surroundings and the items we deal with on a daily basis we are observing issues of contrast.
Whether it be the contrast of the shirt and pants you have chose to wear, your socks and shoes or the colors you have chosen to paint your home. Observing contrast is part of our daily lives whether we realize it or not. When it comes to determining which types of contrast are more appealing to some than others, it all boils down to what I would simply refer to as personal preference.
For example I know some folks who wear bright red and white clothing who spike their hair and color it purple. An excellent example of high contrast. While on the other hand I know other folks who seem to blend into society on a more conservative note who take more of a low contrast approach. Which is better? Answer: Neither. Each individual has their own personal preference which they enjoy because of what they perceive to be more appealing, in their own minds.
Let us now attempt to take this concept of contrast into the world of diamonds and examine how the labs are looking at it but more importantly how contrast plays a vital role in diamond appearance and whether this aspect appeals to your personal preference or not.
What has been taught to date.
There has been a slight mixture of teaching on the subject of contrast which I'd like to tie together here.
The results of my own studies and observations with diamonds have led me to conclude a concept of contrast between facets reflecting back light vs facets reflecting back shadows (more commonly referred to as "head obstruction"). This type of contrast is best observed in hemisphere type lighting which can be seen in this photograph depicting this lighting condition. The brighter the lighting environment the more intense the reflections of white will be. If the light conditions were weaker the facets would reflect back light at a weaker intensity as depicted in the 2nd graphic. Note the whites are not as bright as they are in the 1st graphic.
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contrast illustration 1
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contrast illustration 2
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As we begin to explore AGS' definition of contrast in their nomenclature they too have arrived at this same conclusion and for good reason as it is a lighting condition common to the viewing environments in many people's lives. For those who prefer this appearance in these conditions, Hearts & Arrows optical symmetry becomes an important aspect to them if it is their personal preference. H&A however is only one of many contrast appearances we will be exploring in this tutorial as we learn all the metrics that comprise a cut grade. You may find your personal preference lies with a different appearance and also a lighting condition you may deem as being more important.
Material has been published on the subject of contrast by international gemologists and students of gemology which I would like to share in this chapter. This will help give a further understanding of how AGS has arrived at its conclusions and their determination of this metric besides the incredible man hours and science that has gone into their own research. My first recollections discussing this are with 2 fellow peers. Mr. Garry Holloway FGAA, DipDT, JAA Appraiser, inventor of the Ideal-Scope and Michael Cowing, G.G, FGAA director of ACA Gemological Laboratory in Maryland.
In Garry Holloway's articles on the subject (another one here) (a short read, please check these out) he features the following graphics and introduces another element to the equation of contrast. The role of light leakage and how it plays on contrast as well.
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contrast illustration 3
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If you look at contrast illustrations 1 and 2 you can plainly see all of these elements of contrast at play in both our photography and also the simulated graphic (courtesy of MSU's DiamCalc software). Below is that diamond moving before your eyes in that environment and very much duplicates what you will see in real life in a lighting environment that I would describe as low ambient/hemisphere type lighting. We will be making more of these comparisons as we progress in our study as contrast plays a vital role in all the optical metrics we will be exploring.
AGS definition of Contrast:
When explaining the use of their ASET technology, when viewing diamonds through this device the color blue is represented by the head obstruction we discussed above causing the shadows/darkness we saw in contrast to the bright areas.
Here is an ASET image of a Hearts & Arrows type diamond and AGS's comments regarding contrast.
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Graphic
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AGS Nomenclature
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"Blue is the area that typically produces dark contrast areas in the diamond. It is usually caused by the observers head. The amount and distribution are important. For a well made round brilliant this is about 18% blue."
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For a full discussion of ASET technology please review our chapter on the subject here.
Further commentary in AGS' nomenclature regarding contrast state:
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"Your head and body, the surrounding environment and different lighting environments can produce contrast. Your head is the usual cause of contrast.
"Humans are 'hard wired' to detect edges. Contrast provides us with these edges. Here's an example of a shape with virtually no contrast and the same shape with contrast. Humans find the image on the right to be very appealing compared to the left. Other researchers on diamond cut have pointed out that the presence of contrast enhances our perception of diamond brilliance."
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While not a separate metric in the GIA system, GIA notes that contrast is quite important. In their article in 2004 (http://www.gia.edu/pdfs/cut_fall2004.pdf) on page 21, they say, “Aspects of overall appearance seen as positive features include facet reflections of even, balanced size, with sufficient contrast between bright and dark areas of various sizes so that some minimal level of crispness (or sharpness) of the faceting is displayed in the face-up pattern. There are also appearance aspects that are considered negative traits: For example, a diamond should not display a fisheye or large dark areas in it pattern.” In attending a talk by them, they explained that they built formulas, based on observations that determined when patterns reached certain “thresholds.” For example, when a certain pattern became heavier or darker, the grade then goes down.
In closing this chapter on contrast I would like to reference a graphic I had used in an illustration from another tutorial plus one by Mr. Holloway on the subject wherein we both use the example of a checkerboard.
Here are 3 graphics demonstrating both Garry's and my own teaching using the checkerboard.
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Example of a flat white placard with No Contrast (Holloway).
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Bright Reflector Contrast (Holloway, myself)
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Low Reflector Contrast (Rhino)
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In summary:
Contrast within diamond can be caused by reflections of shadows (usually head obstruction) & leakage as contrasted to areas reflecting light bright areas.
Since contrast impacts appearance in all lighting environments we will be discussing, exploring and viewing their impact in these various environments and their impact upon the metrics used to determine cut grades. Now that we have given ample explanation of contrast and the elements involved that can cause it, let's proceed to the next chapter on Brightness.
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