The black obstruction visible under the table facet of this GIA Excellent cut diamond makes it look dull and lifeless. In this instance, the obstruction is likely due to poor optical precision and the lower girdle facet length.
The red arrows indicate the presence of obstruction. In contrast, the green triangle shows how clear the sections under the table facet should appear.
Be sure to watch the video for this 2.03 carat, F-color, VS-2 clarity, GIA Excellent cut diamond from Blue Nile. In that case, you'll be able to see how inconsistently light is reflecting throughout this diamond.
Consequently, the gemological laboratories don't take optical precision into account as part of the grading process. That is why we say that AGS Ideal and GIA Excellent cut's overall grades might not be good enough.
The Obstruction of Light:
At first glance, the proportions of this diamond appear to be within the range we recommend. In this case, the 40.8-degree pavilion angle should produce a high volume of light return. At the same time, the 35-degree crown angle should create a virtual balance of brilliance and dispersion.
However, the 85% lower girdle facet length is well beyond our preferred range between 75 and 80%. As a result, the arrows pattern looks think and spindle-like. Simultaneously, it is the likely culprit that is obstructing the table facet.
Of course, the lower girdle facet length is simply the most prominent piece of the puzzle. The odds are that there is also inconsistency in the facet shape, size, and alignment. Unfortunately, we don't have the reflector scope images necessary to make that determination.
The Word of the Day is Ob·struc·tion:
"And today boys and girls, we’re going to learn all about obstruction. For the sake of this discussion, that’s going to be anything that gets in the way of our light."
Imagine that you are riding in that bus today that drives through Mr. Rogers neighborhood. When it goes to turn the corner, that car parked on the corner is going to obstruct its ability to turn right.
Now don’t be a wiseacre and point out that the bus is really a trolley car. Just nod your head politely and go along for the ride.
Feel the frustration that you would feel if you were "on the bus” trying to make that right hand turn in the midst of all that obstruction!
Good. Now imagine that you’re about to drop several thousand dollars on a shiny bright rock. That shouldn’t be too hard!
Choosing the Best GIA Excellent Cut diamond:
This tutorial on obstruction in diamonds is inspired by an inquiry from a client just like you. He sent me an email asking me to evaluate the following (8) GIA Excellent cut round diamond from James Allen.
In addition, he indicated that cut quality was most important as he understood that it has a direct impact on optical brilliance.
Here Are the Eight Diamonds:
Right Click on the Links & Follow Along:
Select "Open link in new tab” and start to mull over the diamond details.
I found myself thinking: "I really need to write a tutorial on obstruction!”
So, I sent an email to the client requesting permission to use his inquiry as a blog post. Permission granted. Here you go.
Obstruction: it’s all about taste.
Before we get too far along, I want to offer a bit of a disclaimer. As with selecting the right wine, choosing the right diamond is largely a matter of personal taste.
A while ago, one of my friends purchased the majority share of an upstart winery in Oregon. I attended the grand opening, where the Vintner proudly poured me a glass of wine as I walked through the door.
I took a sip and told him that it was very nice. Based upon his reaction, you would think that I just took a whiz off the bow of the Queen Mary in the middle of Easter Sunday brunch.
You can hear my best friend laughing, right?
"Nice?!?!” the Vintner exclaimed — "It’s just Nice?!?!”
And then he whipped around and grabbed a glass of wine sitting among a throng of pre-poured glasses sitting on the table behind him. "Here try this is” he said rather snidely.
"Yea, that’s nice too,” I said with a great big smile, full well knowing that I’d somehow managed to light this guys fuse.
"W-e-l-l-l-l-l then,” he said, "you just go right ahead and drink that swill because it’s about $10 a bottle! And this wine which is ‘nice’ is more than $200 per bottle!”
Clearly, I have no appreciation of fine wine.
However, I am a connoisseur of finely cut diamonds and a leading authority on light performance. Ahem.
Working 35+ years in the trenches enables one to develop an appreciation for the subtle nuances of diamond sparkle. Whereas the average Joe buying a diamond might not share my keen insight into the differences. Especially since those are created by subtle shifts in proportions and facet structure.
The reality is that at this junction point in time, you might not know what you like in a diamond. That is perfectly all right, and to be expected. Throw me into the depths of your chosen career and I’ll be feeling a little lost also.
For the sake of argument, I’m going to assume that you want to buy a vibrant diamond. One that exhibits the highest volume of light return and a virtual balance of brilliance (white sparkle) and dispersion (colored sparkle).
This is why I’m going to explain everything in great detail and point out some things within the images provided by James Allen. Many thanks to the client who gave me permission to use this content as a blog post! I’m sure that a lot of people will benefit from reading it. On with the show!
James Allen Diamond Reviews: GIA 1172891992
This 1.02 carat, G-color, VVS-2 clarity, James Allen True Hearts diamond is the one that I like best out of the group. It is also the only GIA Excellent cut round diamond of the group that is within tolerance of my selection criteria.
Let’s begin by evaluating this diamond and then discuss why I do not recommend buying the other options. The total depth of this diamond is 62% which is just a hint beyond the scope of my preferred range. However, the reality is that this is not going to affect the light performance.
That is a minor technical factor that is of no consequence in this particular instance. The extra depth is due to girdle thickness, and the girdle edge is within my preferred range. But for the record, I prefer to keep the total depth between 59 – 61.8%.
The pavilion angle of 40.6 degrees should produce a high volume of light return. The 35.0 degree crown angle produces a virtual balance of brilliance (white sparkle) and dispersion (colored sparkle).
The combination of the 75% lower girdle facet length and a higher degree of optical precision should produce broad-spectrum sparkle. That is the sparkle that tends to be larger in size, and which is bolder, brighter and more vivid, than what standard ideal cut diamonds exhibit.
Identifying Obstruction in Diamonds:
If you look at the clarity image provided below, you’ll see that there virtually no signs of obstruction. For your reference, the region you want to focus on are the triangular sections highlighted in pink.
These represent the eight subsections of the table facet where obstruction is likely to occur. Notice how the triangular sections that appear between the arrows are clear and open.
In this instance, there are no significant signs of obstruction or light leakage. Obstruction is the term used to describe the asymmetrical triangular shape reflections that can appear in between the arrows in the middle to upper zone.
Consequently, obstruction is not the same thing as clustering that can occur at the base of the arrows pattern. To be clear, obstruction occurs at the mid-point of the arrow shafts. While clustering may be present at the base of the arrows.
What causes obstruction in diamonds?
Obstruction can result from slight differences in the size, shape, or alignment and indexing of the facets. Diamonds that exhibit crisp and complete patterns of hearts and arrows tend to exhibit very little obstruction.
Whereas diamonds with a lot of obstruction almost always exhibit irregular patterns of hearts and arrows. That is why "optical precision” should be a critical factor in your diamond selection process.
Higher levels of optical precision produce crisper patterns of hearts and arrows, that results in less obstruction. Minimizing the amount of obstruction visible under the table facet, increases the volume of light return.
Obviously, more light and open space under the table facets produces brighter, whiter looking diamonds.
Obstruction vs Clustering & Interference Figures:
Keep in mind that obstruction is not the same as the clustering you may see at the base of the arrows. Obstruction occurs higher up in the middle to outer edge of the inverted triangular sections that appear between the arrows pattern.
While clustering may be visible at the base of the arrows, where it might create some depth of field in the form of contrast.
The green arrows in this photograph, point to where clustering is slightly visible at the base of the arrows in this 1.315 carat, H-color, VS-1 clarity, James Allen True Hearts diamond.
Star facet length is a factor that affects the visibility of obstruction, clustering, or interference figures. In this case, the star facet length is 54% and that likely contributing to the clustering effect. In a perfect world, the optimum range for star facet length is somewhere between 45 – 50%.
However, it’s more likely going to be somewhere between 40 – 58% and that is probably fine. The reality is that a little bit of clustering can actually add depth of field in the form of contrast.
With that in mind, I don’t spend too much time worrying about star facet length. Rather I prefer to look at the images provided, to determine what effect the minor facets are having. If you don’t see a bunch of obstruction under the table facet, there is probably nothing to to worry about.
Obstruction of Light in Diamonds:
The 1.01 carat, G-color, VVS-1 clarity, James Allen True Hearts diamond on the leftr shows a little bit of clustering. Don’t be thrown off by the million pink arrows I’ve added to this picture, because I realize that this is probably what you’re going to do.
Consequently, it seems like every time I try to teach people about obstruction, they automatically focus on the clustering.
With that in mind, I’m trying to get you to see the difference and realize that some things which may seem similar, are often very different things.
Thus, it is important to remember that all of the black areas that you see within a diamond are not obstruction. The arrows pattern appears to be black here because the pavilion main facets are reflecting back the darker color of the camera lens.
There are also reflections of light that exhibit different degrees of contrast brilliance throughout the diamond. Contrast brilliance is an optical effect that creates a depth of field and makes diamonds more attractive.
For the purpose of this tutorial, you should focus only on the triangular sections referenced in the first image. Focus on the small black areas of obstruction that are evident at the base of the arrows pattern.
Why Brian Gavin says "It's all in the hearts"
To better understand what is creating the obstruction visible within this diamond, we need to examine the hearts pattern:
Focus on are the differences in the space indicated along the edge of the hearts by the pink arrows. The space around some of the hearts is larger than it is around some of the other hearts.
As a matter of fact, it is that tiny amount of extra space around the hearts that creates the obstruction visible at the base of the arrows pattern.
The green circles are intended to draw your attention to the tip of the hearts.
Notice how the tips of the hearts appear to be bending ever so slightly.
This is an optical illusion created by slight differences in the length of the lower girdle facets.
Cutting Hearts & Arrows diamonds:
The diagram provided by Brian Gavin which appears to the left shows the pavilion facets of a diamond outlined in orange. The pavilion main facet that is located in the twelve o’clock position has been colored green.
Imagine that the green color is light which is striking the pavilion main facet. That light reflects across the diamond and is split into two halves by the pavilion main facet located in the six o’clock position.
Then that light reflects over on to the lower girdle facet located on either side of the pavilion main. In doing so, it creates one half of the heart shape.
This process repeats itself throughout the entire facet structure of the diamond, creating the 360° pattern of hearts. Any variance in the structure or indexing of the lower girdle facets will create hearts of different sizes, shapes, or which twist at the tips.
Now there are a couple of things that I want you to realize. The first of which is that each of these diamonds is cut by hand. Meaning that each turn of the diamond upon the diamond cutting wheel is positioned by hand.
Thus there really is no such thing as a perfectly cut diamond. There are simply degrees of perfection and our intent is to narrow down the options and select the best diamond currently available.
Back to Our James Allen Review:
Before I get completely distracted, let’s review the proportions of this 1.01 carat, G-color, VVS-1 clarity, James Allen True Hearts diamond. The 40.6 degree pavilion angle should produce a high volume of light return.
While the 34.0 degree crown angle is likely to produce a hint more brilliance, as opposed to a virtual balance of brilliance and dispersion. The 75% lower girdle facet length should produce sparkle that is larger in size and bolder in appearance.
This is a good option for people who might prefer a bit more white sparkle. As opposed to a virtual balance of white and colored sparkle. Understand that this is one of those matters of personal preference that I eluded to earlier.
Obviously, my preference is to see a virtual balance of brilliance and dispersion. In that case, I’m more likely to only consider diamonds with a crown angle between 34.3 – 35.0 degrees.
What Heavy Obstruction Looks Like:
To better understand the visual effects of obstruction, we need to see an example of what that looks like. This 1.02 carat, G-color, Internally Flawless clarity, GIA Excellent cut round diamond from James Allen is just what the doctor ordered.
Just look at all that obstruction visible under the table facet between the arrows pattern on the left. You can imagine the effect that such a heavy volume of obstruction is likely to have upon the light return of a diamond.
My impression of diamonds such as this is that they face-up much darker. This is because there seems to be less light reflecting up from under the table facet. From my perspective, this is not really surprising.
Keeping Everything In Proportion:
A round brilliant cut diamond is simply a three-dimensional model comprised of 58 mirrored surfaces. If you position the mirrors at the right angle and with great precision, you will achieve the highest levels of light performance.
If you don’t take the time to angle the pieces correctly and you haphazardly slap things together, the diamond is going to leak light and exhibit moderate amounts of obstruction.
In this instance, the pavilion angle of 41.2 degrees is too steep for my liking. Plus, it is combined with a 43.5% pavilion depth. That just so happens to be "the critical tipping point” where light begins not to strike fully off of the pavilion facets. The angle of the pavilion facets simply isn’t in the optimum position to return the maximum volume of light. To maximize light return, be sure to keep the pavilion angle between 40.6 – 40.9 degrees.
The crown angle of 32 degrees is also extremely shallow. In my experience, this is likely to produce a higher volume of brilliance or white sparkle. However, this will usually be at the expense of dispersion, which is the colored sparkle commonly known as fire. This is definitely not the type of diamond that I believe we’re looking for.
Do You Really Need an ASET Scope Image?
People often ask me whether they really need an ASET image when buying a diamond. The answer depends on what degree of precision and light performance you seek. An ASET image enables us to determine how effectively a diamond is processing the light that is available.
An ASET scope image demonstrates how evenly light is reflecting throughout the diamond. It also provides us with clear insight as to the degree that a diamond might be leaking light. Thus I do prefer to have ASET scope and other reflector scope images when evaluating diamonds.
However, there are some tell-tale traits that I’ve learned to identify without an ASET scope. Obviously, this is one of those things that comes with experience. Look at enough diamonds through an ASET and Ideal Scope, you’ll begin to pick up on the patterns.
Do You See What I See?
For example, the odds are that all of the light brown, blue, and black areas along the edge of this diamond, will show up as green in an ASET scope image. In a perfect world, all of these areas would show up red in an ASET scope image.
The color green indicates secondary brightness, while red represents primary brightness. I’ve highlighted the areas that I suspect will show up as green under an ASET Scope with green arrows to make them easier for you to identify.
One or two green sections along the edge of a diamond are nothing to be concerned about. However, I’m seeing more than a few sections that are likely to go green along the edge of this diamond.
Is a 35.5 Degree Crown Angle Too Steep?
This 0.91 carat, F-color, VVS-1 clarity, GIA Excellent cut round diamond from James Allen shows minimal obstruction between the arrows pattern. Thus we know that it’s likely to face-up bright and white and exhibit a good light return under the table facet.
Remember that I’m using pink arrows to point out the obstruction, it appears to be very slight in this diamond. The green arrows indicate areas that I believe will show up green under an ASET scope.
We know from experience that the 40.6 degree pavilion angle should produce a high volume of light return. The 35.5 degree crown angle is a little steeper than my preferred range of 34.3 – 35.0 degrees. The likely effect of the steeper crown angle is that the diamond will exhibit a hint more dispersion (colored sparkle, fire).
Is GIA Cutting Corners by Rounding?
In the example shown above, the difference in the degree of brilliance and dispersion is primarily a matter of personal preference. However, it is also important to consider that the GIA rounds off the crown angle measurement to the nearest half a degree. In that case, the crown angle might actually be something like 35.3 to 35.74 degrees.
I have no issue if it is on the lower side of things. However, I find that diamonds with crown angles close to 36 degrees perform better when viewed under pin-fire type lighting. This means that diamonds with steep crown sections look best when viewed under jewelry store halogen lighting, or candlelight and firelight.
Unfortunately, I find that diamonds with a 36 degree crown angle tend to "flatten out” when viewed under diffused light. That is the type of lighting that most of us live and work under in this modern age.
This diamond might be perfect for somebody who prefers a warmer looking diamond. Understand that I’m not talking about the temperature of the actual diamond color. But rather the visual impression of the diamond itself and the overall sparkle factor.
People who prefer diamonds that give off a cooler, crisp looking sparkle are more likely to prefer hearts and arrows diamonds. This is why I prefer super ideal cut diamonds with proportions in the middle of the spectrum.
People who prefer diamonds that look "warmer” often prefer diamonds with a slightly steeper crown section. Once again, it’s all a matter of personal preference. I’m just the tour guide.
Is a 36.5 Degree Crown Angle Too Steep?
Remember that diamonds with a crown angle around 36 degrees tend to look their best when viewed under pin-fire type lighting. However, diamonds with a 36 degree crown angle also tend to flatten out when viewed under diffused lighting. This 0.86 carat, F-color, Internally flawless, GIA Excellent cut round diamond from James Allen has a crown angle of 36.5 degrees!
So what effect might the steeper crown height have upon the visual properties of this diamond?
Let’s take a look:
Clearly we can see that there is a moderate amount of obstruction visible between some of the arrows. Now you can see how the obstruction reduces the amount of visible white space under the table facet.
But you might also notice how the color of the triangular sections under the table facet appears to be getting darker. You can also see how the width of the arrows appears to be getting thicker and bulkier looking. This is an optical effect created by the offset of the crown and pavilion sections.
Now you might like the optical properties of this diamond if you spend a lot of time under pin-fire type lighting. However, this is definitely not the kind of diamond that I would choose for myself.
Because I prefer the consistent light performance provided by diamonds cut to the center range of proportions. After all, they’re going to look exceptional in practically every lighting environment.
Black vs Translucent Arrows Pattern:
Notice how the arrows pattern on the 0.90 carat, D-color, VVS-2 clarity, GIA Excellent cut round diamond from James Allen below are translucent. The arrows pattern of all the other diamonds we’ve looked at thus far have been black.
That is because they are properly reflecting back the dark color of the camera lens. One of the factors that could be creating this effect is the 43.5% pavilion depth.
Remember that I consider that to be "the critical tipping point” where light begins not to fully strike off the pavilion facets. The pavilion main facets of the diamond are not at the best angle to reflect the maximum volume of light.
Thus this diamond does not exhibit good contrast brilliance. Diamonds that exhibit these optical properties always look a bit flat, dull, and lifeless to me.
This might also be due to there being a bit of azimuth shift or facet yaw within the facet structure of the diamond.
Note that the article referenced contains images that illustrate the concept. However, it just became obvious to me that I need to finish that article. Whoops!
Getting Packed Steep & Deep:
Here again, in the example above the diamond has a really steep crown angle of 36.5 degrees. As a matter of fact, I want you to notice the correlation between the total depth and the visible outside diameter.
Because steep crown and deep pavilion sections add unnecessary carat weight to a diamond. This takes the form of carat weight which is hidden in the depth of the diamond.
That means that you are paying a premium for diamond carat weight that is not visible as outside diameter! So not only are you paying for carat weight that you’re not visually benefiting from in terms of the visible outside diameter.
In that case, you are paying for carat weight that is literally robbing you of the sparkle factor! Why do diamond cutters produce "steep, deep diamonds” such as these? "It’s all about BIG Profits baby!”
Is 32.5 Degree Crown Angle Too Shallow?
The 0.86 carat, F-color, VVS-1 clarity, GIA Excellent cut round diamond from James Allen shown below is actually quite bright. In fact, there appears to be a lot of light coming up from under the table facet. That is because of the 40.8 degree pavilion angle and there is not very much obstruction.
Consequently, the crown angle of 32.5 degrees featured on this diamond is pretty shallow by my standards. That is likely to produce a higher volume of brilliance, but it will be at the expense of dispersion.
Now that’s perfectly all right if you happen to prefer to see more white sparkle in a diamond. But we know that I personally happen to prefer diamonds that exhibit a virtual balance of brilliance and dispersion.
In that case, I'm more likely to select a diamond with a crown angle between 34.3 – 35.0 degrees. One thing is for certain, I would definitely not buy a diamond with these proportions without seeing reflector scope images first.
The odds are that these proportions are going to result in some light leakage. I want to be able to gauge the extent of any light leakage that may be present before making a decision. This statement is actually true of all diamonds, even those with center range proportions.
GIA Excellent with 36 Degree Crown Angle:
This 0.85 carat, F-color, Internally Flawless, GIA Excellent cut round diamond from James Allen below has a 36 degree crown angle. You know from what I’ve shared with you thus far that I’m not a fan of diamonds with steeper crown heights. I want a diamond that is going to perform well in all lighting environments, not one which is likely to flatten out under diffused light.
By now it is much easier for you to pick-up on the moderate to heavy obstruction that is apparent under the table facet.
And you are also able to see how the steeper crown height is making the center region of the diamond face-up a little darker.
Although this diamond has a pavilion angle of 40.8 degrees, it’s just doesn’t to be "popping” for me.
Once again, the steeper crown height is adding to the total depth of the diamond. At a whopping 62.7% you’re paying for a lot of carat weight that is hidden in total depth.
Where to buy the Brightest Diamonds:
In my experience, these are the best performing brands of diamonds:
Consequently, the last four on this list tend to be cut to a higher degree of optical precision that reduces obstruction. Remember that there is a direct correlation between the precision of the hearts and arrows pattern and the degree of obstruction visible under the table facet.
Benefits of Hearts & Arrows Precision:
Crisper patterns of hearts and arrows result in reduced amounts of obstruction.
Tighter proportions produce brighter-looking diamonds and a virtual balance of brilliance and dispersion.
Reflector Scope images enable us to judge optical precision:
Take advantage of my free Diamond Concierge Service if you want help finding a diamond. Or maybe you simply have a simple question that you want answered in the form of a really long tutorial? Please leave a comment below.