Diamond weight is expressed in carats. Carats are further divided into 100 “points” so that a 0.90 ct diamond may be called a 90-pointer, a 0.88 carat diamond an 88-pointer and so on. The word ‘carat’ has roots in ancient times, when diamonds were compared against carob beans by traders in the Ottoman Empire.
Carat weight is NOT size. Diamonds with the same carat weight can have smaller or larger vertical spreads, depending on the geometry of their cut. This is no different than two people weighing the same, but one is taller and the other is wider.
Shallow and Deep Diamonds
Carat weight influences price more than any other factor so the goal of mass-manufacturers of diamonds will always be keeping as much weight from the rough crystal in the diamond as possible. This is even more important than the beauty of the final product since bigger diamonds bring bigger money. The result is millions of diamonds on the market which sparkle under bright lights (all diamonds do) but have average performance in normal lighting.
Fine Make Diamonds
Diamonds cut with a goal of beauty and high performance are proportionate, with critical angles and precision-cutting that will promote the best brightness, dispersion and scintillation, even in normal and romantic lighting. Such high performing diamonds appear larger than diamonds of normal cut quality because they have edge-to-edge brightness, not just a bright center. Because it costs more to fine-tune the rough to this degree of beauty, these diamonds are far more rare than shallow and deep diamonds and cost a bit more.
The example above, with a spread of 6.30 millimetres was cut slightly “deep” and will look smaller than a proportionate one-ct diamond, spreading closer to 6.50 mm. The example on the left above has greater spread, but was cut “shallow” and will lack the critical angles necessary for performance, reducing edge-to-edge brightness and potentially increasing dark zones.
Carat Weight and Laboratory Grading
Carat weight is standardized and repeatable around the world. What is not repeatable is the assessment of whether a diamond is too shallow or deep to promote optimum light return.