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The most extensive research into how light acts in a princess cut has been conducted by the American Gem Society (AGS). Unlike past less-accurate cut grading systems, the AGS system detects the actual light performance produced by each individual diamond. The grade is then categorized on a 0-10 scale, with “0” being the best. Most untrained eyes will see little brilliance difference in the top four grades.

The Big Problem

The most common problem with princess cut engagement rings is that almost all are cut too deep. Deep diamonds are not as brilliant as they could be. Worse, a deep diamond looks small. For instance, you may buy a ¾ carat diamond, and in fact, it will weigh ¾ carat. However, if it’s cut deep (as most are) it will only have the face up size (dimensions) of a ½ carat. Cutters leave extra weight on diamonds to make more money. It’s like buying cheap hamburger which is loaded with worthless fat. You may think it’s a great price per pound, when in actuality you are just buying unwanted bulk.

Below is an invaluable chart showing proper average length and width millimeter measurements for princess diamonds that are not cut deep.

Carat Length and Width
.40 4.1 mm x 4.1 mm
.50 4.5 mm x 4.5 mm
.60 4.7 mm x 4.7 mm
.75 5.0 mm x 5.0 mm
.85 5.2 mm x 5.2 mm
.90 5.4 mm x 5.4 mm
1.00 5.6 mm x 5.6 mm
1.25 6.0 mm x 5.6 mm
1.50 6.4 mm x 6.4 mm
2.00 7.0 mm x 7.0 mm

Rule: To know if a princess diamond is deep, you must get the millimeter measurements of length and width from any seller. Otherwise you will get a small-looking unbrilliant.

Does Proper Size Guarantee Brilliance?

Finding a princess with the proper size for its weight reveals it is not a deep cut. However, proper size is just the first step. Even though a princess measures correctly, several other things could be wrong with the cut, any of which could kill the brilliance. Measurements are only one of eleven crucial factors needed to grade a princess cut diamond.

The Princess Cut Grade

Just like the other diamond grading categories of color (yellowness), carat (weight), and clarity (inclusions), the crucial area of cut also has a defined, measurable grade. Both the size and beauty are evaluated in a comprehensive, 0-10 scale which measures the actual brilliance of each diamond tested.

If no cut grade is offered while comparing different sellers, then the cut is most likely poor. A savvy buyer will not buy at places incapable or unwilling to offer a verifiable princess cut grade based on the AGS 0-10 scale. This system is accessible to any jeweler, and is the only system recognized for assessing princess cuts. (GIA has no princess system).

A Note on Squareness

Squareness by itself does not create brilliance, just a pleasing symmetry. Rectangular princess diamonds can still be beautiful, even though they cost slightly less. Most princess diamonds are not perfectly square. The AGS cut grade system takes squareness into account. Noticeably rectangular princess cuts are downgraded, but princess cuts that are close to square get no deductions.

Rule: In order to purchase a great princess cut engagement ring you can be proud of for years to come, do not allow any seller to leave out the cut grade. You wouldn’t allow them to leave out carat weight, or clarity, so insist on cut grade as well. Remember, cut is absolutely crucial to the look of the diamond and makes up as much as 40% of the cost.