Updated: May 4
This is a quick "How To" Diamond blog, part of a series of articles that will help you become better at measuring and estimating carat in Diamonds. This method works great for loose or mounted diamonds. (Bezel setting might be tricky)
You will need:
A Round Diamond or Gemstone to measure
Tweezers (for loose diamonds)
Millimetre Gauge (Check my recommendations at bottom of the page)
Pen and Paper for Notes
Calculator or your mobile phone
To determine a round brilliant’s average girdle diameter, you have to measure it from one edge of the girdle directly across to the opposite edge.
Diamonds and Gemstones are always measured in millimetres, so use a millimetre gauge for accurate measurements.
As you probably will see, no gem is perfectly round, so take several diameter measurements. I normally take four.
The Two measurements Method
Add the smallest and largest diameters and then average them by dividing the result by two.
For example: if the minimum girdle diameter is 4.58mm and the maximum is 4.63mm, then
Average Girdle Diameter = (4.58 + 4.63) ÷ 2
= 9.21 ÷ 2
When using the average girdle diameter to calculate a diamond’s proportions, use the result to the thousandths place.
The Four measurements Method:
If the first measurement is 4.58mm, the second is 4.59, the third is 4.63 and the fourth is 4.60, then
Average Girdle Diameter =
(4.58 + 4.59 + 4.63 + 4.60) ÷ 4
= 18.4 ÷ 4
The more measurements you take the more accurate the calculation is
Here are my recommendations on Tools for measuring Diamonds and Gemstones
My Absolute Favourite is the Presidium Gem Computer Gauge. I can't tell you enough how amazing this tool is, it's top quality.
This pocket computer can do outstanding calculations in a fracture of a second. You can calculate a carat of a Gemstone from the list, using the Specific gravity of the pre-installed Gems, you can also add your own lists. It also comes with a PC software
Maybe for the more serious Gem enthusiasts as it comes at a cost of SDG $449 (Approx. GBP £259 – EUR€308 – USD $324 – Currency date 04/05/2022)
Presidium Website LINK TO PRODUCT
This is a tool that you only buy once in your career.
The other Gauge is more affordable, you can pick them up from ebay at under £10.00