You can use the diamond mm to calculate the carat weight of a mounted stone if the exact weight is unknown. Here are the GIA formulas and tables for calculating the carat weight of mounted diamonds:
Round Brilliant Cut Diamond MM Formula: the average diameter x the average diameter x the depth x 0.0061 = the approximate carat weight. To determine the average diameter of a diamond add the length plus the width and divide it by two.
Keep in mind that these formulas are for approximation purposes only and that the actual weight of the diamond might be slightly different when unmounted.
If you’re staring at the formula and thinking "@#*! say what?!?!” it works like this. Think of the surface of the diamond as if it were a clock and measure the distance from the twelve o’clock to the six o’clock position, let’s say it measures 6.48 mm.
Then measure the distance from the nine o’clock to the three o’clock position, let’s say that measures 6.51 mm. Add 6.48 to 6.51 and you get 12.99 which you then divide by two (the number of the measurements taken) and the average diameter is 6.49 mm.
The Diamond MM to Carat Weight Formula in Plain English:
Measure the diamond from the table facet (flat facet on the top) down to the culet (bottom point), let’s say that is 3.96 mm and you’ll have the numbers required to run the formula. For instance:
6.49 x 6.49 = 42.1850 x 3.96 = 167.052 x 0.0061 = 1.01 carats
The measurements used for this calculation were taken from an AGS Diamond Quality Document which indicates that the carat weight described on the lab report as measuring 6.48 – 6.51 x 3.96 mm was 1.014 carats. Well I’ll be, would you look at that!
Go ahead. Say it. "Well I’ll be, would you look at that.”
Formula for Calculating Diamond MM to Carat Weight for Oval Brilliant Cut Diamond:
The average diameter x average diameter x depth x 0.0062 = approximate carat weight.
Formula for Estimating the Carat Weight of a Heart Shape Brilliant Diamond:
Length x width x depth x 0.0059 = approximate carat weight.
Formula for Estimating the Carat Weight of a Triangular Brilliant (Trillion) Cut Diamond:
Length x width x depth x 0.0057 = approximate carat weight.
Formula for Estimating the Carat Weight of an Emerald Cut, Princess Cut, and Quadrillion Cut Diamond:
Length x width x depth x adjustment factor (see below) = approximate carat weight.
Adjustment Factor for Emerald, Princess, Radiant cut diamonds:
First determine the diamonds length to width ratio by dividing the diamonds length by its width and determining its relationship to 1.00.
For example, if a diamonds length is 7.22 mm and the width of the diamond is 3.90 mm and it's depth is 2.50 mm, then the length to width ratio would be calculated by dividing 7.22 by 3.90 which would equal 1.851282.
The result is then rounded to 1.85 and expressed as 1.85 to 1.00 or stated for appraisal purposes as 1.85:1.00.
Once the length to width ratio is determined, use the adjustment factors below in the aforementioned formula to estimate the diamond's approximate carat weight.
Marquise Cut Brilliant Diamond Formula: length x width x depth x adjustment factor.
In the example above, the diamond’s length to width ratio is 1.85:1.00 which does not appear on the table above.
Since the ratio is close to 2.00:1.00, you use 0.0110 as your adjustment factor.
To make your estimate more accurate, interpolate an adjustment factor between those for 1.50:1.00 and 2.00:1.00. Here, 0.0098 would be good.
Pear Shape Brilliant Cut Diamond Formula: length x width x depth x adjustment factor.
So there you have it, if you want to verify the carat weight of a mounted diamond all you have to do is ask the jeweler for a micrometer or the diamond's dimensions and use the formulas above. Note that approximate weights for common diamond sizes can be found within the user guide included with most micrometers.