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An introduction to diamonds

Carat Weight



Shapes of diamonds


Next steps, tips

Cut is by far the most difficult to measure of the 4 Cs. In an ideally-proportioned diamond, all the light entering the diamond from the top should bounce within the diamond and reflect back through the top, giving the stone maximum brilliance and fire. If the stone is too shallow or too deep, some light will escape through the bottom of the diamond, giving the appearance of shadows when viewed from the top.

"Ideal" Cut Deep Cut

It's easy to see that the deep-cut diamond shown above may have a higher carat weight, but it is clearly the less desirable stone! Many jewelers will not discuss cut proportions unless the customer specifically asks. A stone rich in carat weight but poorly proportioned can be deeply "discounted," giving the buyer a false impression of a great deal.

Ideal Proportion Metrics. In order to assess how well a given diamond is cut to ideal proportions, you will have to measure the diamond. If the stone has a GIA, AGS, or other certificate, the measurements will be on the certificate. Note that it is common to use the relative measurements of the diamond, expressed in percentages, rather than the absolute measurements which are typically in millimeters (mm).

In 1919, Dr. Marcel Tolkowsky empirically calculated the ideal proportions of a round diamond as part of his Ph.D. thesis in Mathematics. According to Tolkowsky, an "ideal cut" has the following characteristics:

• Round in shape, and brilliant-cut (58 facets)
• Depth percentage: 59%
• Table percentage: 53%
• Crown height percentage: 16%
• Pavilion depth percentage: 43%
• Girdle thickness: Medium and even all the way around the diamond
• Symmetry: Perfect
• Perfectly aligned and formed facets
• Very small or absent culet

It should be obvious that finding a Tolkowsky diamond is an expensive and time-consuming task. To further complicate matters, recent GIA studies demonstrate that, in contrast with Tolkowsky's 53% specification, the brilliance of a diamond varies significantly with many other specifications beyond the table.

Let your eyes decide. While the debate continues over the question of the "ideal" specifications, our suggestion is that you use the above numbers as guidelines only, and let your eyes be the final judge. This is why it's critical to see and feel the diamond you purchase, at a reputable local jeweler.

Other Factors Related to Cut. Aside from the right overall proportions and a shape that suits the owner's personal taste, a diamond's value and intrinsic beauty are governed by the workmanship of the diamond cutter. Diamond cutter workmanship includes facets, girdle, cutlet, symmetry, and finish.

Facets. Facets are the flat polished surfaces of a gemstone. When looking at diamonds under a microscope, compare each facet with the ideal form presented here. For example, a round diamond has exactly 58 facets -- 33 above the girdle, and 25 below.

When looking at actual stones through your local jeweler's microscope, refer to the diagram above and try to identify and rate each of the facets. Extra, absent, or deformed facets decrease the value of the stone.

Girdle. The girdle of a diamond is the middle, or fattest part, of the diamond, measured from "extremely thin" to "extremely thick":

Thin girdles are prone to chipping. Extremely thick girdles hide a lot of weight, so you end up paying for a heavier diamond which doesn't look that big. The ideal is an even, medium girdle. This applies to all shapes EXCEPT for the tip of the pear shape, in which a thick girdle is preferred to make it less prone to damage. When examining the girdle, make sure it is well-faceted and polished. Some diamond cutters will omit this step to save time and to avoid removing weight from the stone.

Culet. The culet is the bottom part of the diamond, where the pavilion comes together in a point (see the diagram above, in "facets" section). Culets are rated as "none", "small", "medium", and "large." The ideal is "none," but if an otherwise perfect stone has a medium or large culet, it may still be a worthwhile purchase.

Symmetry. Check the following aspects of the stone to make sure it is completely symmetrical:

  • Make sure the culet is precisely in the middle of the stone when viewing it from the bottom.

  • Verify that the pavilion and crown have the same angle all the way around the stone, and are not bowed out or concave.

  • If you're purchasing a heart shape or any other fancy shape, be sure there are no deformities. For example, some heart-shaped diamonds will have unsightly asymmetrical lobes. Some princess shape or radiants are not quite square.

  • Inspect the table to make sure it is flat and symmetrical.

Finish. There's no hard and fast rule to judge the finish of a stone. However, the more a diamond cutter polishes the stone, the more carat weight (and value) it loses. The cutter's goal will be to polish the stone just enough to satisfy the potential customer. Polishing also removes surface flaws related to clarity, so an "IF" diamond could potentially be turned into an "FL" simply by polishing it.

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Other great resources for quality diamond information:

Diamond Review
Diamond Helpers

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