We received this inquiry from a client who wants to know the difference between COSTCO and Blue Nile Diamonds: "I'm looking at COSTCO Diamonds and Blue Nile. I like 1-1.5 carats and think we have a budget of 8K. But, we might be able to go up a little more than that. I want a good diamond and like the pave petite round setting. I think that is the name. I am new to researching diamonds. Is this price range possible? Here are a few COSTCO Diamonds that I'm thinking about right now." -- Tina G.
The diamonds from COSTCO below are the ones that this client is asking about. As with all of the diamonds that I review, we will consider them on the basis of my selection criteria.
It's important to realize that there is only so much that you can tell about a diamond by the numbers. Which is why I prefer looking at loose diamonds, so that we can use reflector scopes to judge optical precision.
An ASET Scope reveals how well a diamond makes use of the available light in the room. It will also enable you to determine whether light is reflecting evenly throughout the diamond. While an ideal Scope will help you determine the extent of light leakage.
In addition, you'll want to use a Hearts and Arrows Scope to verify optical precision. Which is the consistency of facet shape, size, and alignment throughout the diamond. The combination of these factors will have a direct impact on the volume of light return and sparkle intensity.
Unfortunately, most box stores like Costco don't provide the reflector scope images necessary to judge light performance. As a matter of fact, neither do most online retailers like Blue Nile. Which makes it difficult to judge the degree of light leakage and/or the light performance of their diamonds. On the other hand, diamond cutters like Brian Gavin provide those images for your peace of mind.
The characteristics of this diamond engagement from Costco are as follows:
Let's begin by looking up this diamond using GIA Report Check. That way we'll have all the relevant details, such as proportions and clarity characteristics. According to the GIA, the details of report #5123030941 are as follows:
If you're familiar with my selection criteria, then you know that this Costco diamond doesn't make the grade. In the first place, the crown angle of 32.5° is too shallow. Generally speaking, you want the crown angle to be between 34.3 to 35.0 degrees.
Comparatively, the crown angle of this diamond is likely to produce a lot of brilliance (white sparkle). At the same time, it will likely be at the expense of dispersion (spectral bliss/fire). While a crown angle between 34.3 and 35° will produce a virtual balance of sparkle factor.
By the same token, the table diameter of 59% is too large, especially with the shallow crown angle. In my experience, this combination of large table diameter and shallow crown angle will negatively impact light performance. For one thing, it will probably make the arrows pattern look thin and spindly. In addition, it's likely to create a lot of obstruction under the table facet.
Under normal circumstances, the pavilion angle of 40.8° will produce a high volume of light return. However, each piece of the diamond works in conjunction with the other parts. Hence the joke about one plus one equals three.
I'm sure you can see why I can't recommend this Costco diamond. Given these points it should be clear that the parameters for the GIA Excellent cut grade are too broad.
How does the Costco diamond above compare with what I was able to find at Blue Nile? In like manner, the crown angle of this 1.13 carat, G-color, VVS-2 clarity, GIA Excellent cut diamond from Blue Nile is a bit shallow. However, the crown angle of 34 degrees is preferable and should produce better sparkle.
According to the GIA, this diamond has a total depth of 59.3% and a 58% table diameter. The pavilion angle of 40.8 degrees should produce a high volume of light return. While the 34° crown angle produces a good balance of brilliance and dispersion.
At the same time, the 34° crown angle is still likely to produce a little more brilliance than dispersion. Which means that this diamond will probably exhibit a little more white sparkle than fire. By the same token, the difference in sparkle between a crown angle of 34 - 34.3 degrees is slight. The odds are that most people are not likely to notice the difference without some coaching.
The proportions of this diamond are within the spectrum for the AGS Ideal-0 cut grade. On the contrary, the proportions of the diamond from Costco are not within the ideal range.
In addition, I don't know how the IGI came up with the value of this diamond ring from Costco. However, I assume that it represents their estimate of full blown retail and includes the cost of the ring. At the same time, the Blue Nile diamond is selling for $11,301 and offers better light performance.
All right, so if you're keeping score, it's currently Blue Nile: 1 / Costco: 0
According to the GIA, this Costco diamond has a total depth of 60.0% and a 59% table diameter. In addition, the diamond has a 33° crown angle that is offset by a 41 degree pavilion angle. The girdle edge is medium to slightly thick and faceted and the culet size is none.
This diamond has an overall grade of GIA Excellent and proportions within the Ideal spectrum. Be that as it may, the crown angle of 33 degrees is much shallower than I prefer.
In spite of that fact, some jewelers think that a shallow crown is a good offset for a steep pavilion. Obviously, I'm not one of those people because I know that this combination has drawbacks. In the first place, the pavilion depth of 43.5% is too steep to produce exceptional light return. As a matter of fact, it's the critical tipping point where light begins not to optimally strike the pavilion facets.That means this diamond is likely to leak a lot of light.
Unfortunately, Tina didn't provide me with the Prices for these Costco diamonds. For this reason, I don't know how they compare with Blue Nile diamond prices. Be that as it may, this 1.27 carat, G-color, VVS-2 clarity, GIA Excellent diamond from Blue Nile has better proportions.
This Blue Nile diamond has a total depth of 61% and a 57% table diameter. The pavilion angle of 41 degrees is still steeper than I prefer. However, the 43% pavilion depth will produce better light return than the 43.5% on the Costco diamond.
At the same time, the crown angle of 34.5° should produce a virtual balance of brilliance and dispersion. With this in mind, this one should produce better light return and sparkle than the Costco diamond. The current price is $13, 862.00 for this GIA Excellent round diamond from Blue Nile.
The score is now Blue Nile: 2 / Costco: 0
The proportions diagram for this GIA Excellent round diamond from Costco is to the left. As can be seen, the pavilion depth is 43.5% once again. Remember, this is the critical tipping point where the volume of light return begins to drop. With this in mind, we obviously want to avoid that 41°/43.5% pavilion angle/depth combination.
On the positive side, the 34.5° crown angle should produce a virtual balance of brilliance and dispersion. Consequently, it really doesn't matter because the 43.5% pavilion depth is a deal killer in my opinion. As an illustration, it's like buying a 100 watt lightbulb that only delivers 1000 lumens instead of the standard 1600.
Not only that but the total depth of 62.1% has a hidden cost which most people don't realize. In the event that the total depth of a diamond is too deep it can negatively impact the diameter. In other words, the visible outside diameter of your diamond shrinks as the total depth increases. Obviously, this is why recommend that you keep the total depth between 59 - 61.8% for round diamonds.
At the present time, this is the best comparison to the previous Costco diamond: 1.12 carat, H-color, VVS-2 clarity, GIA Excellent round from Blue Nile.
To the left is the clarity photograph for this diamond from Blue Nile. This diamond has a total depth of 61.3% and a 57% table diameter. The 40.6° pavilion angle should produce a high volume of light return. At the same time, the 35° crown angle should produce a virtual balance of brilliance and dispersion.
The visible outside diameter of this diamond is larger as a result of the shallower total depth. This diamond measures 6.68 - 6.72 mm, so it has an average outside diameter of 6.70 millimeters. While the 1.13 carat from Costco measures 6.66 - 6.68 mm, which gives it an average outside diameter of 6.67 millimeters. As such, the smaller Blue Nile diamond faces up larger.
For this reason, I prefer a total depth between 59-61.8% for round brilliant cut diamonds. For information on fancy shape diamonds, refer to this article on calculating proportions. By the way, this Blue Nile diamond is currently selling for $8,654.00 which seems better than the Costco price.
The current score is Blue Nile: 3 / Costco: 0 (which means something in baseball).
Under the circumstances you might be thinking that Blue Nile diamonds are better than Costco diamonds. Although this may be true, it also might not be. Because it's important to realize that this game is rigged.
In the first place, the Costco diamonds which I'm reviewing were chosen by Tina. With this in mind, there is a high probability that they will not meet my selection criteria. On the other hand, the odds are high that my search will yield better results. After all, I'm diamond buyer by profession with 30+ years of trade experience.
In the event that you search Blue Nile, you'll see plenty of options with proportions similar to Costco diamonds. As a matter of fact, you'll find that most of their diamonds do not meet my selection criteria.
To put it differently, when I search for diamonds the goal is to cherry pick the best options available. Which means I'm not going to mention Blue Nile diamonds that aren't better than the ones from Costco. Which is common sense, right? LOL, I guess I cheated!
DOH! But hey, it's important to keep things in perspective. In the first place, these are the Costco diamonds that interest my client. To that end, it's not my fault that these aren't diamonds that I would pick.
At the same time, try to look at the situation from my vantage point. To begin with, the client chose these diamonds from Costco, not me. By the same token, the client indicates that she's also considering diamonds from Blue Nile. With that in mind, it's in the best interest of my client for me to provide her with better options. She can always use the insight herein to select other Costco diamonds that might perform better than these.
In the event that you're looking for the most spectacular looking diamond possible, here's a little advice. The first thing to remember is that diamond cut quality dictates light performance. Which means that the combination of proportions and optical precision will determine the intensity of sparkle and light return.
To put it differently, you're not going to get the best performance from a diamond with less than ideal make. Although this may be true, the proportions of a diamond are only one piece of the puzzle. As a matter of fact, the degree of optical precision might have more influence on sparkle factor.
Which is why you might consider dropping down in clarity so that you can enjoy better performance. It's a common misconception that higher clarity diamonds look better than lower clarity diamonds.
This 1.245 carat, I-color, VS-2 clarity, Brian Gavin Signature diamond will look the same as a VVS-2 without magnification. At the same time, the proportions and higher cut quality of this diamond will deliver better sparkle factor. The 40.8° pavilion angle will produce a higher volume of light return. At the same time, the 34.8° crown angle will produce a virtual balance of brilliance and dispersion. Not to mention the higher degree of optical precision which will produce sparkle that is more vivid and intense.
Although this may be true, there is nothing like photographic evidence to seal the deal. In view of that reality, take a look at the reflector scope images on the diamond details page. The ASET and Ideal Scope images indicate the highest volume of light return. Another key point is how evenly light appears to be reflecting throughout this diamond. As can be seen by the even distribution of hue and saturation across the ASET Scope image.
In addition, the diamond exhibits a picture-perfect pattern of hearts and arrows. Which is a clear indication of the higher degree of optical precision. Comparatively speaking it can take up to 4x longer to polish an ideal cut diamond to this degree of perfection.
Immediately upon reading this post, Tina asked me to run a new search for Blue Nile and Costco diamonds. Obviously, this time we will conduct the search using my selection criteria to ensure success.
With that in mind, you can see how I set the search parameters on Blue Nile to the left. We'll be searching for diamonds with a total depth between 59 - 61.8% and a table diameter between 53 - 58%. Inasmuch as this will narrow down the field of possibilities it does not guarantee success.
As can be seen by the examples above, the parameters for the GIA Excellent rating are broad. Unfortunately, Blue Nile does not enable us to search by crown or pavilion angle measurements. Which means that we will have to open up the details page for each diamond in the results. In view of that fact, you'll need to right click your mouse over each line on the list. Then choose the option to open the page in a new tab of your browser.
This will enable us to see whether the crown and pavilion measurements are within the range we are looking for. Remember that you're looking for diamonds with a crown angle between 34.3 – 35.0° offset by a 40.6 - 40.9 degree pavilion angle. Just close the page in the event that any of these Blue Nile diamonds have proportions outside this range.
Take a look at this 1.28 carat, G-color, VS-2 clarity, GIA Excellent cut diamond from Blue Nile. As you can see on the diamond grading report below the proportions meet my selection criteria.
This diamond has a total depth of 60.3% with a 58% table diameter. The pavilion angle of 40.8° should produce a high volume of light return. At the same time, the 34.5° crown angle should produce the brilliance and dispersion you're looking for.
The 75% Lower Girdle Facet length should produce and arrows pattern with a good balance. The combination of proportions and this LGF tends to produce sparkle that is larger in size and bolder in appearance.
By the same token, LGF in the range of 80-82% is likely to produce sparkle which is smaller in size and less intense. Although this may be true, it's important to realize that the GIA Laboratory rounds this measurement off to the nearest five percent. Which means that an LGF of 78% will appear as 80% on a GIA diamond grading report.
Given these points, this diamond should exhibit a dazzling personality that is bright and sparkly! Seeing that the portions are in the "sweet spot" for the zero ideal cut proportions rating. In fact, the proportions of this diamond are spot-on within the Tolkowsky ideal cut spectrum.
Under those circumstances, it's not surprising that this diamond also scores well on the Holloway Cut Adviser. As a matter of fact, it scores 1.3 Excellent for Light Return, Fire, Scintillation, and Spread. To clarify, the total depth of 60.3% is the reason this diamond scores well on the HCA for spread. In the event that the total depth was 60.4% and higher, the HCA score would be very good for spread.
This other 1.28 carat, G-color, VS-2 clarity, GIA Excellent cut round diamond from Blue Nile meets my proportions criteria.The 40.8 degree pavilion angle should produce a high volume of light return. At the same time, the 34.5° crown angle should produce a nice balance of brilliance and dispersion.
The total depth of 61.5% and 55% table diameter are in line with my expectations. By the same token, the 75% lower girdle facets should produce sparkle that is larger in size.
Although this may be true, I do not recommend that you buy this diamond. The reason is because of the inclusions within it. According to the key to symbols under the plotting diagram this diamond contains the following clarity characteristics: crystal, cloud, knot, feather, and needle.
Consequently, a knot is an included diamond crystal that breaks the surface. In this particular instance, the knot appears on the lower plotting diagram. Look for the small red and green circles just to the left of the pavilion main facet in the 12 o'clock position. Knot inclusions pose a potential durability risk as you will discover below.
In the event that a diamond contains any of the inclusions above we reject it automatically with prejudice. On the condition that these types of inclusions might pose a durability risk. Which is not to say that they are a problem, but rather that they might be. Under those circumstances, we prefer to err on the side of caution. Especially when there are so many other viable options to consider that do not contain these inclusions.
This 1.29 carat, G-color, VS-1 clarity, GIA Excellent cut round from Blue Nile doesn't quite meet my proportions criteria. As shown on the diamond grading report below, the crown angle is only 34 degrees. From time to time, I'll consider diamonds with this crown angle, but only if the pavilion angle is spot-on.
The reason is because the GIA round off the crown angle to the nearest half a degree. Which means that a crown angle of 34.2° will appear as 34° on the diamond grading report. At the same time, a crown angle of 34.3° shows as 34.5° on a lab report from the GIA.
By the same token, a crown angle of 33.8° will appear on a GIA diamond grading report as 34 degrees. With this in mind, considering diamonds like this represents a bit of a gamble. Obviously, you only want to consider this diamond if the crown angle is on the high side of 34 degrees.
Truth be told, I usually don't give diamonds like this a second glance because I don't like to gamble. By the same token, I have the benefit of seeing thousands of diamonds in-person. In other words, I know firsthand the difference that proportions have on light performance.
At the same time, most people don't have the experience of comparing diamonds side-by-side. Which means that they are buying blind for the most part. Given these points, I strongly recommend adhering to the proportions in the article 15 Seconds to Diamond Buying Success.
As can be seen by the examples above, diamond proportions are a critical part of the equation. As a matter of fact, you should use proportions to narrow down the field of possibilities.
By the same token, knowing the proportions of the diamond will only get you so far. Consider the proportions of this 1.24 carat, E-color, VS-2 clarity, GIA Excellent cut diamond from Blue Nile as an example.
According to the GIA this diamond has a 40.8 pavilion angle, thus the light return should be spectacular. To say nothing of the brilliance and dispersion that the 34.5° crown angle should produce.
The lower girdle facet length looks like 78% to me, which the GIA rounds off to eighty percent. With this in mind, this diamond should exhibit a high volume of light return. At the same time, it should produce a virtual balance of brilliance and dispersion.
So, why does the diamond look dark in the middle?
The Hallway Cut Adviser (HCA) gives this diamond a score of 1.3 Excellent due to the proportions. In this case, the diamond scores excellent for light return, fire, and scintillation. As you may recall, the HCA scores diamonds very good for spread unless the total depth is <60.3%.
Although this may be true, I'm going to demonstrate why diamond proportions are only the beginning. Which is why Gary Holloway describes the HCA as a diamond elimination tool. With this in mind, you should use tools like the HCA to narrow down the field of possibilities. And then immediately look at the reflector scope images to eliminate the options that perform poorly.
From time to time, I'm able to find reflector scope images for Blue Nile diamonds even though they don't provide them. This is one of the hidden benefits of my status as a trade member. Here's the Ideal Scope image for this 1.24 carat, E-color, VS-2 clarity, GIA Excellent cut diamond from Blue Nile.
The light pink semi-transparent sections under the table facet indicate light leakage. At the same time, it's easy to see that light is not reflecting evenly throughout this diamond. This type of revelation can be shocking to people on the condition that they assume GIA Excellent means Excellence. The simple truth is that every nuance of diamond grading represents a spectrum of possibility.
The ASET Scope image for this 1.24 carat, E-color, VS-2 clarity, GIA Excellent cut diamond from Blue Nile tells a similar story. In the same way as the Ideal Scope image, this ASET reveals light leakage under the table facet.
In addition, we can use ASET to determine how evenly light is reflecting throughout the diamond. On the contrary, we can see how unevenly this diamond reflects light. In other words, the colors red, green, and blue, do not reflect throughout this diamond evenly. Which is an indication of the degree of optical precision rather than proportions. By the way, be sure to read the article "What do the different colors of ASET mean?"
These reflector scope images are the most compelling evidence in support of their necessity. Is important to realize that there is more to buying a diamond than whether it's from Blue Nile or Costco. After all, neither of these companies produce diamonds, they simply resell them.
Here's another key point that I want you to remember: Each of these reflector scope images has a specific purpose which the design reflects. While the scopes might reveal light leakage or consistency to varying degrees, they are not interchangeable. With this in mind, you should always be certain to get both an ASET and an Ideal Scope image.
By the way, I'm not going to try to weed through all the options for Costco diamonds. In view of the fact that they don't seem to offer loose diamonds online.
At the same time, I find the search engine for Costco Diamonds to be a frustrating and mind-numbing experience. In light of that situation, I will be happy to review the details for any Costco diamond. However, only if you do the legwork and send me the diamond rating report number.
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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