"How important is diamond culet size? You specify a preference for a culet size of GIA “none” or AGS “pointed” in the five minute diamond buying guide."
"Does a very small or small culet size make that much of a difference in a diamond's appearance? I'm shopping for a diamond engagement ring online.
“The Astor by Blue Nile and James Allen True Hearts diamonds seem to offer the best balance of quality and price.”
“Neither vendor seems to have anything that matches your selection criteria right now. However, I’m hoping you can help me select the best option from the list provided.”
What Is the Culet of a Diamond?
The culet is the bottom facet of the diamond that appears to be a point. However, it is clearly an octagon-shaped facet under higher degrees of magnification.
We recommend a culet size of AGS "pointed" or GIA "none" because it is the most aesthetically pleasing. Consequently, the aforementioned AGS and GIA descriptions are different ways of expressing the same thing.
A diamond culet size of GIA none or AGS pointed will not be visible through the diamonds' table facet. In contrast, a small or very small culet is likely to appear like a small white circle or black dot.
Conversely, if you're going to spend the money for a higher clarity, then it seems counterintuitive to buy one with a small culet. First, the larger culet size will make it seem like there is a visible inclusion in the middle of the table facet. In other words, a very small culet looks similar to a diamond crystal.
Second, you're paying a premium for an ideal cut diamond and a larger culet size reflects a lack of skill and precision. Consequently, I feel like cutters use the larger culet size to cover up pavilion facet alignment issues. Perhaps they couldn't achieve excellent symmetry without covering their tracks?
Blue Nile Diamond reviews: GIA 5166801160
This 1.12 carat, G-color, VS-1 clarity, GIA Excellent cut diamond from Blue Nile does not meet my selection criteria. However, we will review it because it's at the top of your short list.
In this case, the 40.8 degree pavilion angle should produce a high volume of light return. However, the 42.5% pavilion depth is a mismatch and more appropriate for a 40.6 degree pavilion angle.
In contrast, a pavilion depth of 43% is more appropriate for a 40.8 degree pavilion angle. The mismatch raises a red flag in my mind as a diamond buyer.
The 35.5 degree crown angle of 35.5 degrees is steeper than the range of 34.3 - 35-degrees that I recommend. That might be contributing to the obstruction under the table facet. It might also cause more light leakage along the edge of the table facet. However, we need an ASET or Ideal Scope image to determine whether or not that is an issue.
If you look at the clarity image, you'll see that the arrows pattern is a little thin. That results from the 80% lower girdle facet length and tends to produce sparkle that is smaller in size. Conversely, smaller sparkle might seem more brilliant, but that's only because it's more difficult for our eyes to disperse into colors.
Evaluating the Hearts' Pattern for Optical Precision:
It's essential to use hearts and arrows scope images to judge optical precision. That is the consistency of facet shape, size, and alignment from the perspective of 360-degrees.
In this case, the red arrow highlights were light is not reflecting evenly off the lower girdle facets. You can also see how the reflection of light looks more like lawn darts instead of hearts.
Notice how the tip of the heart in the 12 o'clock position is bending to the left. That is an indication of Azimuth Shift. In other words, there is difference in the indexing of the pavilion facets.
In that case, light is not reflecting evenly and the difference in size is evident in the lack of consistency. Consequently, this has little or nothing to do with culet size.
Optical Precision is Essential to Performance:
This illustration from Brian Gavin Diamonds shows how light reflects off the facets to create the hearts' pattern. The pavilion facets appear with an orange outline for your reference.
In this case, light reflecting off the main pavilion facet at 12 o'clock appears in green. The light reflects off the main pavilion facet across to the other side of the diamond. At which point, the opposing pavilion main splits the light into two halves.
That light then reflects off the lower girdle facet on either side of the pavilion main and creates half of the heart shape. As that pattern of light reflection continues around the stone, it combines to create eight perfect hearts.
Any variance in the size and shape of the facets, or the alignment will alter the shape of the heart halves. In other words, the length of the light reflecting off the facets will be different lengths if the lower girdle facets are different sizes.
Consequently, the difference in facet length is what makes the heart tips seem like they're bending and twisting. In reality, the optical effect results from the difference in the length of the light reflecting off the facets. In contrast, we could simply say that the cutter lacked the skill to really nail it.
Yeah, I Said It. Sorry, Not Sorry.
Look. I know that I'm not the most popular diamond buyer in La La Cutting Land. There are many reasons why the cutters refer to me as the Diamond Nazi, Golden Child, and The Resident Snob.
First, I favor blunt truth over the adage of not saying anything if you can't say anything nice. In that case, I'm going to be honest and say when I don't like something about a diamond.
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” photographs 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.
James Allen True Hearts diamond review: GIA 1197545735
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?
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