Calculating the Carat Weight of Diamonds
This month’s Bite-Sized Diamond Facts, is all about determining carat weight without the use of scales.
Diamond carat weight is always calculated under laboratory conditions, using finely calibrated scales. Through their journey from the mine to the retailer it is customary to weigh diamonds to a thousandth of one carat (three decimal places) and then round this to a hundredth of a carat i.e. two decimal places. The use of formulae will usually give an approximation to the carat weight and not a precise carat weight to work from.
It may be of some interest to learn that scientific formulas can be applied to determine approximate carat weights for diamonds. From Round Brilliant cuts to Marquise diamonds, specific formulae utilise stone measurements to provide a good idea of weight. This is not something that typically would be used, since as noted above, a true weight is determined by the use of highly sensitive and accurate scales. If the weight is already known, one of the “Golden Rules” is to verify the diamond weight, especially when diamonds are circulated on approval, upon receipt and return amongst traders.
When Might This Be Useful?
The application of these formulae can be particularly useful when diamonds are already set. Take for example an engagement ring set with a diamond of a fairly substantial size. If we cannot unset the stone, then we cannot weigh the stone. If however we can measure diameter and depth using an instrument such as a Presidium Diamond Gauge, then we can take specific readings to apply the formulae. Along with an appraisal of the diamond for colour and clarity (best done unset for accuracy) an overall idea of the carat weight, quality and value of the diamond can be determined.
Example Formulae for Different Diamond Shapes
For each of the following an adjustment of -2% for a thin girdle and +3% for a thick girdle will be advisable for a more accurate result. There is also a recommendation to add upto 6% depending on the bulge factor of the pavilion of the diamond. 2% if this is only slight.
Round Brilliant Cut Diamonds
Carats = Diameter² x Depth x 0.00609664
Pear Shaped Diamonds
Carats = 0.0059136 x Length x Width x Depth
Marquise Cut Diamonds
Carats = 0.055651 x Length x Width x Depth
Square Diamonds (Includes Emerald Cuts / Princess / Carré)
Carats = 0.0088563 x Length x Width x Depth
If you have a diamond or two, perhaps take a few measurements and try out the formulae. Chances are the results should be pretty close to the weight. For anyone with a little mathematical skill the above formulae can also be switched around to guesstimate an unknown depth, if carat weight, width and length are known.