What Makes a Diamond Pretty?
If you are in the market to buy a diamond, the ultimate outcome you want to achieve in your diamond purchase is a diamond that is pretty and, being such, has value. It is important to understand what factors go into making a diamond pretty, regardless of the grade or diamond certificate description — that is, regardless of the Four C’s. In my book Beyond the Four C’s — What you should Really Know before you buy a Diamond, this is Chapter 1: What Makes a Diamond Pretty. I believe that this is where diamond education should start. So what makes a diamond pretty?
Three factors make a diamond pretty, and all of them must be present for that diamond to be desirable and worth buying. I follow these guidelines when I buy my diamond inventory, and you should follow them, too, when you buy your special diamond. Many diamonds have one or even two of the factors but not the third. For me, that is not good enough. And it shouldn’t be good enough for you, either.
Before we examine these factors, let’s review the parts of a diamond that yield brilliance and fire.

Figure 12. Parts of a diamond
The most important parts of a diamond are (a) the crown angles, the angles created by the crown and the girdle, (b) the pavilion angles, the angles created by the girdle and the pavilion, and © the culet angle, the angle created by the two sides of the pavilion at the bottom point of the diamond where the culet is located. It is the way these three angles interact that gives a diamond its brilliance and fire — by the reflection of light entering the diamond through the crown, reflecting across the pavilion, and exiting back through the crown. (Figure 13) This is the reflection of light we are looking for throughout the entire diamond.

Figure 13. Ideal light reflection
Let’s examine the three factors individually. Each will be described in greater detail later.
The three crucial elements that make a diamond pretty are the following:
1. The physical size of a diamond has to reflect what a well-cut diamond of that carat weight should look like.
2. The diamond has to be brilliant.
3. The inclusions (imperfections) in the diamond have to be pleasing to the eye when looked at under magnification.
First, the physical size of a diamond, in millimeters, should reflect what a well-cut diamond of that carat weight is supposed to look like. A diamond must meet certain physical dimensions for its carat weight in order for it to have the appearance of a well-cut diamond. For example, a 1.00 carat round diamond should measure approximately 6.4 to 6.5 millimeters in diameter. Many 1.00 carat diamonds measure only 6.0 or 6.1 millimeters; this is a diameter for a well-cut 0.80 carat diamond. There is an appropriate millimeter size for each general carat weight for round brilliant diamonds, as well as fancy-shape diamonds. The premise is the same with all diamonds, but it will be easier to understand if I focus strictly on the basic round brilliant.
It may not surprise you to learn that a 6.0 or 6.1 millimeter one-carat round diamond will sell for a lot less than a 6.5 millimeter one-carat round diamond of the same quality. You shouldn’t purchase that 6.0 or 6.1 millimeter round diamond, even at the lower price, because that diamond will not look like a one-carat diamond. A picture is worth a thousand words:

Figure 14. 6.5 mm versus 6.1 mm diamond
Both diamonds in this picture weigh one carat, and they are both the same quality diamond and the same grade on paper. The one on the right is less expensive, but the one on the left is desirable.
The second element that makes a diamond pretty is that a diamond has to be brilliant. The brilliance of a diamond refers to the light and fire that reflects back at you when you look at it. It is a visual, not something you can see from a certificate. Figure 15 shows a well-proportioned diamond on the right and demonstrates the way light reacts with the angles of the diamond to reflect back through the crown. The diagrams on the left and in the center show a shallow and a deep diamond, respectively, and how light refracts out.

Figure 15. How light reflects
Some diamonds have the correct angles and are very brilliant but are still undesirable. These are diamonds with very thick girdles.

Figure 16. Very thick girdle
When a diamond’s important angles have been formed properly, it may look brilliant even if its girdle is very thick, and it will probably cost much less than a similarly graded diamond with a thinner girdle. You may think that it is a great diamond because it is so brilliant, not to mention a great deal because it costs less. However, such a diamond is not pretty because it doesn’t meet the size requirements. It doesn’t look like a one-carat diamond. Its diameter is too small. It has worthless carat weight in the girdle.
Finally, the nature of the inclusions in the diamond has to be pleasing when looked at under magnification. Judging whether the nature of the inclusions in a diamond is pleasing or not may seem to be an easy task. After all, aren’t all diamonds with the same clarity grade the same? That is another myth.
All diamonds with the same clarity grade do not necessarily have the same desirability. Each clarity grade is a range, and every diamond has its own unique characteristics; some are pleasing, and some are not. In fact, a diamond with a lower clarity grade may be more desirable than one with a higher clarity grade because of the nature and location of the imperfections. You can’t just go by the grade. You need a qualified individual to look at the diamond for you and judge the nature of the inclusions.
When I look at a diamond under magnification, I look to see whether the nature of that diamond’s clarity is desirable. If it is desirable, I’m interested. If it is not, I’m not interested in buying it at any price. The same should go for you.
These three factors are what make a diamond pretty, and you need to consider all of them. The measurements, the brilliance, and the nature of the inclusions all have to be right. When they are, you have a diamond worth buying.