Sometimes you need to determine the quality of a diamond, but you don’t have a grading certificate for it. In such a case, you can still grade the stone, but the grades you will come up with will be approximate and are best expressed in terms of grading ranges instead of exact values.
What Diamond Characteristic Should You Grade?
The most important characteristics that are graded in a diamond are its color, clarity, and cut.
Diamond carat weight is measured in carats and, like the above three attributes, is also an important determinant of diamond price.
There are different grading scales for diamond color, carat, and cut, but for this article, we will use one of the most popular standards – the system introduced by the GIA (Gemological Institute of America).
On the GIA scale, diamond color is graded by using letters – from D (the highest grade) to Z (the lowest grade).
The GIA clarity scale consists of the following grades (from highest to lowest): IF, FL, VVS1, VVS2, VS1, VS2, SI1, SI2, I1, I2, and I3.
The cut grading scale used by the GIA includes the following grades: Excellent, Very Good, Good, Fair, and Poor.
How to Grade Diamond Color
Diamond color is graded by comparing the evaluated stone with a set of graded master stones and determining which one of them is closest to the assessed diamond in terms of color.
If you don’t have access to diamonds that you can use as benchmarks to compare your stone against, you can try to determine its approximate color grade by looking at how colorless it looks.
If your diamond doesn’t appear to have a visible yellow tint, then the stone color grade will most likely fall in the D-E-F-G-H range. It’s hard to pinpoint the exact grade further without using graded stones for comparison.
Diamonds that have a barely visible yellowish hue most likely belong to the I-J-K-L-M grading range.
Stones that have some yellow color that is very light but nonetheless visible would likely be graded in the N-O-P-Q-R range.
Diamonds with a clearly visible yellow tint are usually graded with one of the color grades in the S-Z range.
How to Grade Diamond Clarity
Diamond clarity is graded depending on how many natural flaws can be seen within a diamond or on its surface by looking at the stone under magnification or with the naked eye.
If you look at a diamond using a 10x loupe and cannot readily see any visible flaws inside, then the stone’s clarity would most likely be graded in the FL/IF/VVS1/VVS2 range.
The difference between the FL/IF and VVS1/VVS2 grades is that the latter contain some tiny inclusions, but even trained graders cannot spot them easily.
If the diamond you are looking at has some inclusions that are visible under magnification but not to the naked eye, then the stone’s clarity is most likely in the VS1/VS2/SI1 grading range.
Flaws in SI2-clarity diamonds may or may not be visible to the unaided eye, but if they are, they will usually be located towards the sides of the stone.
If the diamond has inclusions that are easily noticeable with the naked eye, then the stone is most likely an I1/I2/I3-clarity diamond.
These stones are more likely to have visible inclusions that are centrally located – i.e., towards the center of the table when the stone is looked at from the top.
As you can see, it is hard to pinpoint the exact clarity grade of a diamond unless you are experienced in gemstone grading.
How to Grade Diamond Cut
To grade the cut of a diamond, you need to evaluate some of its measurements and proportions that are considered important for how the stone reflects light.
Grading cut is a complex process, but you can determine approximate cut quality by looking at some key ratios and characteristics.
One of these important ratios is the diamond’s table percentage, which is calculated by dividing the diameter (for round cuts) or width (for rectangular cuts) of the stone’s table (the topmost facet) by the diameter or width of the girdle (the widest part of the stone).
For round cuts, a good table percentage is in the 52%-60% range. Values between 53% and 58% are considered ideal. Values below 51% or above 64% are seen in cuts graded Fair or Poor.
For rectangular cuts, the table percentage value should ideally be in the 60%-70% range. Proportions that deviate significantly from these values are not desirable.
Another important ratio in cut grading is depth percentage, which is calculated by dividing the stone’s depth (distance from the top to the bottom) by the diameter or width of the girdle.
A good depth percentage for a round cut is in the 57%-64% range, ideally between 58% and 60%.
For most rectangular cuts, an acceptable depth percentage is in the 60%-70% range, but exact desirable ranges vary depending on the particular cut examined.
An important cut characteristic is girdle thickness.
Regardless of cut, the ideal girdle is thin to thick, with extremely thin or extremely thick girdles being undesirable.
Another feature you should pay attention to is the culet – the facet on the very bottom of the diamond.
Ideally, the culet should be small or there should be none. Big culets are not recommended, as they are too visible through the stone’s top.
Estimating Diamond Cut Quality
There are more details that are considered in cut grading, but you can use the above characteristics to make a quick estimate of cut quality.
The rule of thumb is that if all of these characteristics are in their recommended ranges for a particular stone, then its cut can be graded anywhere from Good to Excellent (these grades are most applicable to round diamonds).
The more of the above characteristics fall outside of their recommended ranges for a given cut, the more likely it is to be graded Fair or Poor.
How to Determine Diamond Carat Weight
Carat weight is easy to calculate for a given diamond. Since 1 carat is equal to 0.2 grams, all you have to do is weigh the stone and divide its weight in grams by 0.2.
For example, a diamond that weighs 0.35 g has a carat weight of 1.75 ct (0.35/0.2).