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Cut Proportion

Some content on this page are contributed by Good Old Gold, with thanks.

What is Cut Proportion?

The cut proportion is quite simply the measurement of the sizes and angles of the facets of a diamond. If you go to a jewellery store and ask for the diamond's proportions, you will probably be presented with measurements in the form of the following diagram (or similar):

The question is, how do we determine if the cut proportion is good from this diagram? The following pages will tell you how to determine this.

Importance of Cut Proportion

I cannot emphasis enough on how important cut proportion is.

A well cut diamond reflects a high amount of light. Hence it appears bigger, brighter and prettier. The facets of a diamond function like mini lenses, transferring the light back to the observer and creating an 'optical illusion'. Thus the diamond appears bigger.

Without the benefit of this 'optical illusion', a diamond is in fact very small and not at all attractive. You would be better off with a piece of lead crystal, which is a whole lot cheaper. That is why, ideal cut diamonds attract such a large premium over commercial grade diamonds.

As the difference in size is an 'optical illusion', it is very difficult (if not impossible) to photograph. You should compare some actual diamonds to get the idea.

Essentially, a diamond's cut proportion determines 2 things:

Light Return Which is how brilliant, fiery and sparkly the diamond is.
Structural Integrity Good structural integrity reduces the probability that the diamond will chip or crack when it is being set and under normal wear.

Structural integrity is easy to determine. If the measurements of a diamond falls within certain range, the structural integrity will be affected. For example, very thin girdles are prone to chipping. Fortunately, the reputable gem laboratories do a good job at ensuring structural integrity because they are objective measures. So, if we see a good cut grading on a diamond report, structural integrity is not an issue.

On the other hand, light return is a very subjective concept because beauty is in the eyes of the beholder. That is why it is difficult to determine, and why we will be discussing it at length.

Problems for Consumers

Of all the factors we have considered so far amongst the 4Cs, cut proportion presents the most problems for consumers. We can depend on reputable gem labs to determine the carat, colour and clarity quite accurately. The cut polish and symmetry can also be easily pinned down. However, it is not easy to determine the quality of cut propotions objectively. Hence it was not until January 2006 that the major gem labs (AGS and GIA) were able to grade cut proportions reliably.

Even then, the gem labs do not grade proportion considerations that are largely subjective. For example, a diamond's cut pattern (optical symmetry) is not graded and only extreme girdle painting is being penalised. Read on for more details on these.

Effect of Good Cut Proportions

Quite simply, a diamond that is cut to ideal proportions will have good light return.

There are 2 different types of light that can emanate from a diamond.  One is called brightness and the other fire. The brightness is the return of white light to the human eye, the fire is the return of colored light to the human eye.  These are not subjective terms, but can be scientifically defined and examined.

The movement of both white and colored light within the diamond as it is moved before the observer is properly called scintillation.  A diamond needs to have good brightness and fire before it can have good scintillation.

We are going to examine cut proportions in detail, but before I can do that, I need to explain to you what a diamond with good light return will look like, so that you can form your own critical judgement. Both AGS and GIA use these elements of light return as factors (which they call 'metrics) to determine their cut grades. Else, you may end up with a diamond that looks good only at the store, but not anywhere else.

How topics on proportions are organised

Although cut proportions is a simple concept, you will soon find that understanding it fully can be a challenge. There are many topics on proportions, which I shall try to organise in a logical manner, so that readers can better appreciate the subject.

Elements of Light Return

Since the main objective of good proportions is to get good light return (the other being structural integrity), we have to understand what exactly is good light return. So, over the next few pages, we will explain the elements of good light return in detail. The 3 elements of good light return are:

  1. Brightness with good contrast
  2. Fire
  3. Scintillation

Optical Cut Analysis

Next, we will proceed to assess diamonds optically, using the lightscope. We will introduce this tool and explain how we can use it. When used correctly, the lightscope (or any other tools that employ reflector technology) is a useful tool for predicting the light return. We will explain how we can eliminate diamonds with poor light return using this tool. In fact, The Americal Gemology Society (AGS) uses a similar tool, which they call the ASET, to assess the cut grading of a diamond.

Measuring the Diamond's Facets

After that, will explain how the proportions of a diamond are measured and how these proportion measures affect light return. We will cover all the facets of a diamond that affect its light return, so that you will have a good idea how those measures you read about in diamond reports translate to light return.

What is Ideal Cut?

Finally, we will examine what various people consider as ideal cut diamonds. We will end off with what AGS and GIA considers as ideal and how good their assessments of cut grades are. We will also expose the limitations of these cut grading methodologies so that consumers can make a more informed buying decision by investigating the areas not adequately covered by AGS and GIA in their cut grading.

I am sure you will be able to pick out a winning diamond when you are through with my website.





Next: Elements of Light Return - Brightness & Contrast...