The classic round cut is the most popular of all diamond cuts, and if you need to assess the quality or value of a round diamond, it is important to know how to measure such a stone. This article looks at the most important round diamond measurements and explains how you should go about measuring such a diamond.
The Most Important Round Diamond Measurements
The most important measurements of a round diamond are the diameter of its girdle and table, the height of its crown and pavilion, and the total depth of the stone.
One of the best ways to take these measurements is with a micrometer or a leveridge gauge (a tool that jewelers often use to measure gemstones).
Measuring Diamond Girdle Diameter
If you look at a round diamond from the top, its diameter is the straight line you could draw through the stone’s center, connecting two points that sit opposite each other on the diamond’s girdle.
If you don’t know what the girdle is, this is simply the widest part of the diamond (or, its outer edge).
Measuring Diamond Table Diameter
The diamond’s table is the flat facet on the stone’s top, and its diameter will be smaller than that of the girdle.
The diameter of the table would be measured by taking the length of an imaginary straight line that passes through the facet’s center and connects two points on the table’s sides.
Measuring the Total Depth of a Diamond
The total depth of a round diamond is the vertical distance between its topmost facet (the table) and the stone’s pointed bottom (which could be faceted, in which case the facet would be referred to as “the culet”).
Measuring the Crown Height of a Diamond
The crown height of a diamond constitutes part of its total depth. The crown is actually the top part of the stone, above the girdle.
Diamond crown height is the vertical distance between the top facet (the table) and the girdle.
Measuring Diamond Pavilion Depth
Pavilion depth is also part of a diamond’s total depth. The pavilion is the portion of the stone below the girdle.
Pavilion depth is measured by taking the vertical distance from the girdle to the bottom end of the stone.
What Can You Use Diamond Measurements For?
Now that you know the definitions of the most important diamond measurements, what can you do with them? For one, you could estimate how much the stone weighs.
You could also use the diamond’s measurements to calculate some key proportions and compare them with the benchmark proportions considered ideal for round diamonds.
Estimating Round Diamond Carat Weight by Measurements
You can estimate the approximate carat weight of a round diamond by plugging some of its measurements in the following formula:
Weight in Carats = Girdle Diameter x Girdle Diameter x Total Depth x 0.006
The girdle diameter and total depth values used in the formula should be in millimeters.
For example, if a stone’s diameter is 4.5 mm and its total depth is 2.5 mm, then the estimated weight of this diamond would be 4.5 x 4.5 x 2.5 x 0.006 = 0.30 carats (1 carat = 0.2 grams).
Another quick way to figure out the approximate carat weight of a round diamond is to look at its diameter and see how it compares to the diameter of some common carat weights.
Here is a diameter-to-weight conversion table:
4.1 mm – 0.25 ct
5.1 mm – 0.50 ct
5.8 mm – 0.75 ct
6.4 mm – 1.00 ct
6.9 mm – 1.25 ct
7.4 mm – 1.50 ct
So, for example, if your diamond is about 5.5 mm in diameter, its carat weight is most likely somewhere between 0.50 and 0.75 carats, or around 0.63 carats.
Note that the further your round diamond’s proportions are from those considered standard for this type of cut, the further your estimate will be from reality if you are using this table.
Ideal Cut Measurement Standards for Round Diamonds
There are certain proportions that are deemed to be ideal for round diamonds.
The significance of this benchmark is that the closer a diamond’s measurements are to it, the better the stone will reflect light back to the eye of an observer.
And the more light a stone returns this way, the greater its brilliance.
Diamonds with proportions within the range of the ideal ones are said to have an ideal or excellent cut, and their brilliance is maximized because of the way they are shaped.
Here are the most important ideal diamond proportions (they are calculated as a percentage of the diameter of the girdle):
Table diameter: 53%-57.5%
Crown height: 14.4%-16.2%
Pavilion depth: 43.1%-43.2%
So, for example, if your stone has a girdle with a diameter of 6.5 mm, its table diameter is between 3.45 and 3.75 mm, its crown height is between 0.94 and 1.05 mm, and its pavilion depth is around 2.8 mm, then this diamond can be said to have an ideal cut.