“I’m in the process of shopping for a diamond engagement ring online, and am trying to decide between Blue Nile Signature round diamonds and James Allen True Hearts diamonds, which seem to offer a good balance of quality and price. Neither vendor seems to have anything that matches your preferred range of selection parameters at the moment, but I’m hoping you can help me select the best option from the list provided. How important is diamond culet size? I know that you specify a preference for a culet size of either GIA “none” or AGS “pointed” and indicate that it is the same thing in concept. What difference does a very small or small culet make in the appearance of a diamond?”
The culet is the bottom point of the diamond, it is actually a facet that appears to look like a tiny point. My preference as stated in the article 15 Seconds to Diamond Buying Success is that the diamond culet size is either GIA “none” or AGS “pointed” which are different terms used to describe the same thing.
A diamond culet size of GIA none or AGS pointed will not be visible through the table facet of a diamond. However, as you can see in the clarity photograph for the 1.12 carat, G-color, VS-1 clarity, Blue Nile Signature round diamond, pictured above, a diamond culet size of small is clearly visible as a small black dot in the middle of the table facet. Sometimes a small or very small diamond culet size will show up as a small white circle in the middle of the table facet. Either way is not really good in my opinion as I find that it distracts from the beauty of the diamond.
My preference for diamond culet size is that it be either GIA none or AGS pointed because anything larger than that is likely to appear as a white or black circle or dot in the center of the table facet. It seems to me that if you’re going to spend the money to buy a super ideal cut diamond in a higher clarity so that it will be eye clean, that it doesn’t make sense to buy one with a small or very small culet, since that is going to create a white or black spot in the middle of the diamond that looks a lot like a small diamond crystal.
To be quite frank, I think that cutting a round brilliant ideal cut diamond with a small or very small culet size, reflects a lack of skill and precision by the diamond cutter. It is as if they had to cut a larger diamond culet size at the bottom of the diamond to cover up something in the alignment of the pavilion facets, or maybe they just bumped the stone a bit too hard on the diamond cutting wheel. Either option doesn’t sit well with me, and thus I personally would not purchase a round brilliant cut diamond with a small or very small diamond culet size.
All right, so it’s pretty clear that I’m not a fan of the small culet size featured on this 1.12 carat, G-color, VS-1 clarity, Blue Nile Signature round diamond,. Let’s move beyond that and focus on the proportions. The 40.8 degree pavilion angle would normally be great. However, it is mismatched with a pavilion depth of 42.5%. This is another red flag about the way this diamond has been cut, a more appropriate pavilion depth would be 43% and the pavilion depth of 42.5% would be more appropriate for a pavilion angle of 40.6 degrees.
The crown angle of 35.5 degrees is pretty steep, and well beyond my preferred range of 34.3 to 34.9 degrees. This is likely to make the diamond exhibit a bit more dispersion/fire (colored sparkle) than brilliance (white sparkle) however the 80% lower girdle facet length will help to offset this a bit, because lower girdle facets in the range of 80 – 82% tend to produce pin-fire type sparkle that is smaller in size, which is harder for human eyes to disperse into colored light, thus the sparkle appears to be more brilliant.
Another indication that the light return of this 1.12 carat, G-color, VS-1 clarity, Blue Nile Signature round diamond is not going to be up to my standards, is the fact that half of the hearts pattern pictured in the relative two o’clock and seven o’clock positions is greyed out as indicated by the red arrow. Notice how the heart / lawn dart located in the twelve o’clock position is bent and twisting to the left. This is evidence of major Azimuth Shift. Also take notice of how small the heart is in the relative eleven o’clock position, and how much black space there is between the tip of the heart and the arrowhead that appears beneath it, this is all an indication that the lower girdle facets are different lengths.
In order to better understand what is going on here, take a look at this graphic provided by Brian Gavin Diamonds. It explains how a pattern of hearts and arrows is created in their Brian Gavin Signature round diamonds. The pavilion facet pattern has been outlined in orange. Light reflecting off of the pavilion main facet in the twelve o’clock position has been colored green. As the light reflects off of the pavilion main facet, and is reflected across to the other side of the diamond, it is split into two parts by the pavilion main facet in the six o’clock position, and then the light creates one half of a heart on opposite sides of that pavilion main facet. It requires extreme precision to produce a pattern like this!
In order to produce a pattern of hearts like the one pictured above for the Brian Gavin Signature round diamond, the length of all eight lower girdle facets must be virtually identical. If the length of the lower girdle facets is not the same, then the length of the reflection that creates one half of the heart on the other side will be slightly longer or shorter than the length of the light reflected off of the other pavilion main facet. Thus the two sides of the heart will be different lengths, creating the twisting, bending effect that is visible in the Blue Nile Signature diamond referenced above.
I imagine that you might be thinking something like “But why should I care about whether the diamond exhibits a crisp and complete pattern of hearts and arrows? I’m never going to be able to see the hearts pattern after the diamond is mounted.” That is true, however I’d like to draw your attention to the black blotch that appears at the tip of the yellow arrow, right between the pavilion main facets (arrows) located in the relative two and three o’clock position in this “optical symmetry analysis” photograph provided on the GCAL diamond grading report for this 1.12 carat, G-color, VS-1 clarity, Blue Nile Signature round diamond. Would you care to guess what is causing that black blotch?
Would you believe that it is being caused by that large gap that exists between the heart located in the relative eleven o’clock position? The one that is about half the size of the other hearts, and the arrowhead that appears beneath it? This is the type of irregularity in the distribution of light that is being reflected throughout a diamond, that could be corrected by cutting the diamond to the higher degree of optical precision required to produce a crisp and complete hearts and arrows pattern.
Now I’m wondering whether you noticed anything else in the “optical symmetry analysis” photograph provided for the 1.12 carat, G-color, VS-1 clarity, Blue Nile Signature round diamond pictured above. Like the white spot that appears right in the middle of the table facet, as a result of the small culet? I guess we now know the answer to the questions “How important is the culet size of a diamond?” and “What effect does diamond culet size have upon the appearance of a diamond?” And if you’re still not sure, check out the large white dot that appears in the middle of the light performance image provided above for this diamond by GCAL, that large white dot is because the diamond culet size is small.
With that in mind, I’m going to pass over the other Blue Nile Signature round diamonds that you referenced, which have small or very small culet sizes, since there isn’t much purpose in reviewing diamonds that I would not recommend or buy personally. The only purpose of reviewing the one Blue Nile Signature round diamond was to teach you the importance of diamond culet size and optical precision.
This 1.16 carat, G-color, VS-1 clarity, James Allen True Hearts diamond has an overall cut grade of GIA Excellent. The pavilion angle of 40.6 degrees will provide a high volume of light return. However, I’d prefer that the pavilion depth be more like 42.5% for this pavilion angle, instead of 43% which is better suited for a pavilion angle in the range of 40.8 – 40.9 degrees. The crown angle of 35.5 degrees is beyond my preferred range of 34.3 – 34.9 degrees. However, the crown height is 15% and thus the diamond still has potential in my book.
But only because I’m able to see that the diamond exhibits a pretty decent pattern of hearts and arrows, thus the optical precision of the diamond is very good to excellent. This will help to offset the slight variances from my preferred range of proportions.
Look closely at the hearts photograph featured above, and you’ll see that there is some variation in the size and shape of the hearts. However, it is much less than exhibited by the Blue Nile Signature diamond reviewed above, and thus I’m inclined to choose this diamond over the other.
However, take note that the 80% lower girdle facet length will result in this diamond exhibiting pin-fire type sparkle that is smaller in size. It is also much more difficult for our human eyes (as opposed to a video camera) to disperse into colored light, and thus this diamond might appear to be more brilliant. However, perhaps not so much that anybody would really notice without some coaching… it’s kind of a technical thing, but my preference is for diamonds that exhibit broad-spectrum sparkle, whereas some people prefer diamonds like this which exhibit pin-fire type sparkle. There is no right or wrong regarding the type of diamond sparkle you prefer, it’s kind of like Coke or Pepsi?
If you enjoyed reading this blog post and hopefully learned a little from it, and would like my help finding the diamond of your dreams, or would just like to have a little bit of professional insight about the diamonds you are considering, take advantage of my free Diamond Concierge Service.
Todd Gray is a professional diamond buyer with 30+ years of trade experience. He loves to teach people how to buy diamonds that exhibit incredible light performance! In addition to writing for Nice Ice, Todd "ghost writes" blogs and educational content for other diamond sites. When Todd isn't chained to a desk, or consulting for the trade, he enjoys Freediving! (that's like scuba diving, but without air tanks)
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