In general, there are two reasons why a diamond with a lower carat weight can look bigger than one with a larger carat weight.

1) The larger carat diamond actually has a physically smaller diameter; or
2) The size difference is really just an optical illusion.

It all comes down to cut.

If we’re looking at a diamond that is balanced in terms of light performance and is optimised for spread, then the best-cut diamond fitting this specification will have an optimum diameter such that any change will upset this balance. For convenience, let’s call this hypothetical diamond our ‘perfect diamond’.

There are other diamonds that are optimised for size and these diamonds will have a bigger physical diameter than our perfect diamond. Cutters can achieve this by making certain sacrifices in terms of brilliance, fire, or scintillation. Although this can be seen as a matter of preference, when I normally give advice, I will generally point out to you the differences in contrast pattern between diamonds optimised for size (i.e. 60-60 diamonds) and ideal cut diamonds. Whichever one you prefer will be immediately clear.

But worse, there are some diamonds that are actually optimised for weight retention. These are the diamonds that I do not recommend. If you come to me for help, you will find that I will hardly ever recommend to you a diamond with a 4% girdle. For the same reason I don’t like really high crowns or overly deep pavilions because these are the areas where the diamond can hide it’s weight. Less obvious, but potentially significant, are indexing issues such as painting and digging.

Some of these red flags, like significant digging and large amounts of light leakage, will have drastic consequences to the face-up image of a diamond but others are much more subtle. The problem is that if a cutter pushes to maximise weight retention wherever he can with the objective to receive an excellent cut grade rather than showing restraint and stopping before the point where it will have too much of an effect on the diamond’s light performance.

For premium brands, you can tell a lot about their diamonds from how they display them in their retail stores. I won’t be getting into store lighting today but I’m sure most of you have seen that all diamonds sparkle a ton in brightly lit jewellery stores. You will actually notice that Tiffany doesn’t have as bright lighting as other stores. Yet, their diamonds on display have some of the most sparkle I ever see in a diamond.

Why?

Tricky Cutting

The only conclusion I could think of was that their diamonds are all cut relatively deep. They tend to have smaller tables, steeper crown angles, higher crown heights and their pavilion is also slightly deeper. The ones I’ve seen all had total depths around 62.5% or above. Diamonds that are overly steep-deep are those that prosumers frequently suggest to avoid due to the under table leakage also referred to as the ‘ring of death’. But even slightly steep-deep diamonds can have too much unwanted light leakage.

Tiffany also displays their diamond rings at an angle, I’m guessing specifically designed to enhance their diamonds that are slightly steep-deep. Even overly steep-deep diamonds that exhibit the ring of death can give off a lot of fire and sparkle under the table when viewed at an angle. Remember that I’ve mentioned before in my tutorials that our concept of an ideal cut is based on the face-up view.

In my opinion, Tiffany leverages the fact that consumers believe what their eyes see to make their purchase. Actually, if you take what I consider to be a well-cut unbranded diamond into a Tiffany store, there are some situations where it will under perform their diamonds.

For example, if you’re in a Tiffany store looking down at our perfect diamond from the face-up view, and comparing with a Tiffany diamond that is on display. There is a good chance that our perfect diamond will appear to have less fire and sparkle than the Tiffany. The trade-off is that these steep-deep diamonds will have less spread than our perfect diamond and this is one of the reasons why our small ‘perfect diamond’ may look visibly larger than a larger premium branded diamond side-by-side.

Optical Illusion

The other reason why our perfect diamond may look visibly larger is because it simply appears that way rather than it actually being physically larger. There are several ways that a diamond can appear smaller than it really is. One of the reasons is because of a smaller table. The table is the largest facet and a smaller table can have the effect that the diamond appears smaller.

Another reason is because of poor edge light performance. These diamonds can sometimes sneak into the GIA excellent/AGS ideal cut grades because it is the minor facets that affect this area of the diamond more than anything.

In particular, the upper girdle facets are important but they are not even listed on the lab report. Our perfect diamond will have excellent edge-to-edge light performance coupled with an excellent contrast pattern. This contrast pattern will minimise unwanted dark areas including head shadow/obstruction and leakage issues. If a diamond is cut for weight, often the side effect will be that it sacrifices this light performance on the edge of the diamond where the upper girdles are.

A final reason is because of the mounting. A loose diamond will appear smaller than one in a setting. Also, a diamond with a four-prong setting will appear smaller than a diamond with a six-prong setting. The size of the prong heads also makes a difference. Of course, if the smaller diamond is set in a halo, then the illusion will be even greater.

Conclusion

So now you know why that super-ideal cut diamond you purchased from BGD can look bigger than a larger carat diamond from Tiffany or Cartier. A balanced diamond with a thin to medium girdle in a six-prong setting will look much bigger than a fire/sparkle optimised premium branded diamond with a smaller table and slightly thick girdle in a four-prong setting.