How a Diamond is Cut:
© Sondra Francis

The cut of a diamond determines the brilliance, dispersion and scintillation. Cut makes the diamond beautiful! Well cut diamonds will sell at a higher price per carat than diamonds which are poorly cut. The cutting of a diamond can affect the per carat price up to 25 percent, possibly more, in a given size; but it makes a one hundred percent difference in the beauty of the stone! The term cut encompasses the proportional relationships between the various parts, the quality of the symmetry and the finishing touches of polish, as well as the shape of the stone's outline.

Round Brilliant Cut Diamond

To understand the proportions of a diamond, it is easiest to start with the round brilliant cut. A round brilliant cut diamonds can be cut to maximize brilliance-the white light reflected out of the stone-and dispersion-the spectral color display. A round brilliant cut diamond has fifty-eight facets. If you place the table of a well cut diamond down on a white piece of paper with writing on it, you will not be able to read the words through the diamond! This happens because a well cut diamond is so perfectly able to trap the light that enters to that no light escapes haphazardly: it is all reflected back as brilliance. No other transparent gem material has this ability.

A poorly cut diamond will not do this either. In a diamond with poor proportions light will escape out through the pavilion and optimum brilliance is not achieved. This is especially true with fancy shaped diamonds.

What can be called a well cut diamond can be defined as a stone with defined mathematical relationships of the depth to diameter ratio, the table to diameter ratio, the crown and pavilion angles, the girdle thickness, the symmetry, and the quality of the polish. Yes, it is that complicated!

In 1919 two men, not working together but almost simultaneously, derived the mathematical proportions of an "ideal cut" diamond. In Europe, Marcel Tolkowsky and in the United States, Henry Morse came to nearly identical specifications. An "ideal cut" optimizes the brilliance and dispersion. This does not mean it maximizes both brilliance and dispersion, but combines the two factors to achieve the ultimate of beauty from the diamond. Actually it compromises the optimum aspects of brilliance and dispersion. A diamond cut just for maximum brilliance would lose dispersion and one cut to achieve maximum dispersion would lose some brilliance. Consequently the "ideal cut" is a compromise which combines both factors so the diamond has a balance of each.

Now that you know that the ideal cut maximizes the diamond's beauty you might assume that is the way all diamonds are cut and thus it is what you will buy, right? Unfortunately diamond cutting is more of a business than an art. Very few diamonds have an ideal cut because to achieve these model proportions a lot of weight would be lost when cutting from the rough crystal. The fact of the market place is that diamonds are cut to optimize yield from the rough, not beauty. Of course, if beauty is too severely compromised the diamond is less salable; consequently the cutter compromises yield a little bit with beauty.

What deviation from ideal is acceptable in a cut? Remember, when you are buying a diamond, beauty is your first consideration. A diamond will still be beautiful if the deviations from ideal fall within certain limitations.

The crown angles in well cut diamonds must be within a degree or two of Tolkowsky's 34 1/2 degrees for the crown angle and 40 3/4 degrees for the pavilion angle in order for the light to be trapped with in the diamond to produce optimum brilliance.

In an ideally cut diamond the depth percentage (which is a ratio of depth to diameter) is 60 to 6l percent. If a diamond is too shallow it will generally have a very shallow crown; with a shallow crown there is a great lessening of dispersion. If a diamond is too deep, it will appear to have a dark area in the center, hence brilliance is compromised. If all other aspects are within certain limitations, a diamond can still be quite beautiful if the depth percentage falls in between 57 and 62 percent The table of an ideal cut would range between 53 and 56 percent in ratio to the diameter but few diamonds cut today have tables below 60 percent. If the diamond has a smaller table it has sufficient crown area to display dispersion. The larger the table, the smaller the crown area and the less dispersion the diamond will display. A table between 60 and 64 percent still can have dispersion and fall into our beautiful diamond category.

The girdle on a diamond is an important portion. Ideally the girdle will comprise about one percent of the total depth. If a diamond has a very thin girdle like a knife edge it will be vulnerable to chipping on the edge; serious damage can occur due to chipping. Girdles are rated as thin, medium, slightly thick to thick to extremely thick. Medium is the most desirable. Very thick girdles can add between two and five percent to a diamond's depth; as a result, you will pay for extra weight yet you will have less brilliance. When the diamond is very deep, it will appear to be smaller in diameter than it should be. A very thick girdle often indicates excessive depth.

The symmetry of a diamond is critical to its beauty; a well cut diamond will have it facets cut with symmetrical precision. Lack of symmetry can easily be observed in older cuts of diamonds. Check to see if the pavilion is off center: under magnification view the diamond through the table, the culet will be reflected around the diamond; there appears to be about eight culets instead of one.

The crown of a diamond may not be parallel to the girdle. Girdle thickness may be uneven around the circumference of the stone. Facet shapes can be distorted. The facet relationships are very important to produce scintillation: on a well cut diamond the lower girdle facets match up exactly with the upper girdle facets. When the facets do not match, scintillation is lessened. A symmetrical table is a good indication of overall symmetry. The stone's outline should be nearly round. You can check for this by measuring the diameter of the loose diamond.

When the symmetry of a diamond is compromised in any way, the reflection of light entering the diamond is disturbed and brilliance or scintillation is lessened. Diamonds with symmetry problems are less beautiful and should sell for substantially less per carat than ones without symmetry problems. The quality of polish is essential to the luster of the diamond and is the finishing touch to the beauty. Larger diamonds of decent quality will generally have a reasonably good polish: the cutter would not sacrifice this final detail on a nice stone. However, in lower quality diamonds, cutting and polishing is often unacceptable. Small diamonds, which are referred to in the trade as melee, are sometimes not well polished. Poorly polished diamonds simply do not have optimum sparkle. Poor polish is one reason why some jewelry pieces with small diamond lack their full potential for beauty.

The girdle of a diamond generally is unpolished and may have a dull or granular appearance. On finer qualities of diamonds the girdle will often be polished or faceted. This is an extra step and a fairly expensive addition; but is a finishing touch that adds to a fine quality stone.

The modern round brilliant cut diamond evolved after the invention of the diamond saw in l9l0. Most diamond crystals are bipyramidal octahedrons.

Today these are sawed into two facetable halves. Before l9l0, the crystal was simply ground down to cut one diamond; older cuts of diamonds can easily be distinguished by excessive depth, very small tables, and large culets. Generally the facet relationships in an older cut are quite asymmetrical.

If an older cut of diamond has a squarish or cushion shape it is called an old mine cut. If it has a round shape it is an old European cut. These diamonds have a unique beauty of their own, but lack of symmetry and excessively deep proportions do not allow these diamonds to have their optimum beauty. Consequently the price per carat is generally less than a modern cut of the same weight and quality. Small diamonds or melee, which weigh under .20 carat may either be full cuts which have the full fifty-seven facets of the round brilliant cut or single cuts which have only sixteen facets; single cut diamonds are rarely larger than .05 carat. The term "chips" is usually a misnomer it almost never refers to actual chips. Almost no modern fine jewelry actually has chips in it.

Small diamonds are important accents to fine jewelry. They should be evaluated in the same way as the larger diamonds, except clarity grades are not broken down into the subgrades, the grades are just VVS, VS, SI, and I.

Fancy Shapes

Fancy shaped diamonds include the marquise, pear shape, emerald cut, heart shapes, oval, Trillion, Quadrillion, princess cut and many others. Some of these, such as the Trillion and Quadrillion, have trademarked facet arrangements. Of the fancy cuts, the marquise and the trademarked cuts are generally more expensive per carat than the round brilliant cut. The other fancy shapes will sell for less per carat that the round brilliant.

Fancy cut diamonds are not evaluated according to proscribed mathematical relationships that define the ideal round brilliant cut. Diamonds cut into marquises, pear shapes, heart shapes, and ovals lose optical efficiency just by the virtue of their shape and the maximum potential for brilliance and dispersion is less likely to be achieved.

If you are buying a fancy cut diamond you should scrutinize it very carefully. If the diamond is cut shallow, which is common for the marquises, pear shapes, and ovals, brilliance is sacrificed. When these stones are worn and get dirty, they will look very dull. A well cut but dirty round will look better. With these shapes light cannot be perfectly trapped, so besides brilliance being compromised, a bow-tie may be observable: a dark area can be seen in the center of marquises, pear shapes and ovals. The bow-tie may not be present in very shallow fancy cut diamonds; but these will obviously dull.

Emerald cuts and other cuts with square and rectangular outline shapes will often be cut very deep. Careful faceting arrangements can give rectangular outstanding brilliance. But these diamonds that are very deep will not have the surface area that the weight would suggest.

If you are buying a fancy cut diamond you must objectively look at the stone and judge its overall beauty. Observe the outline shape, is it pleasing? Long, skinny stones will lose optical efficiency. The shape may be pleasing but how is the brilliance? As a shape approaches round the more efficiently the light is reflected. Tables on fancy cuts may be quite large with the result that dispersion is compromised. So look at these very carefully and try to assess their brilliance and dispersion.

Question Private Grading Systems

Some companies may have their own diamond grading system, such as AAA or premier grade or something of that nature. As a buyer you must question what this means. Diamonds of a given size must be evaluated on color, clarity and cut; one designation cannot be used to describe all three of these. If a seller tells you, "All of our diamonds are G color with a VS-1 clarity grade," it should bring some obvious questions to mind. You, the buyer, need enough information to insure you are buying what you want.

Ready to Buy?

So what do you want when buying a diamond? A "D, Flawless with ideal cut," of course. Very fine diamonds with high color and clarity grade with an ideal cut will be visibly more beautiful than those with lesser qualifications. That is why the finer qualities command more money!

Now what can you afford? Most diamond buyers need to make a compromise in order to buy a stone that is large enough to see and still be beautiful. An ideal cut will sell for about a 25 percent premium, but you can find a well cut stone falling into the guidelines described above and still have a beautiful stone.

Color is the biggest factor effecting price. A "D" color diamond is outstanding, but even an "I" or "J" will face up nicely especially if it is set in a yellow gold mounting. Clarity grades that have no structural defects or detracting flaws will look great when the stone is worn.

The important thing is that you know the quality and you make an informed choice. It is essential that you have seen high color, high clarity grade, well cut diamonds and know how they look. Now you have a standard of beauty to compare with. The goal of the diamond buyer is to buy a truly beautiful diamond.

If your budget can afford a fine quality stone, a diamond with a VS clarity grade and "H" color or better, consider buying a diamond with a Diamond Grading Report from the Gem Trade Laboratory of the GIA. These stones have been graded by three diamond experts, they must agree on the final grade. Although the certificate has a disclaimer about this report being a "guarantee," it is pretty good insurance that you are buying the quality stated on the report. This is referred to as a certified stone in the trade. A diamond with a GIA certificate will sell for a slight premium over one with similar color, clarity, and cut, but it may be worth it for the higher qualities. There is little room for doubt concerning quality with a "certed stone." Unless the diamond has been graded by a known professional gemological laboratory, it is not truly "certed."

About the Author

Sondra Francis has scoured every major colored gemstone market in the world since 1978. She was a charter member of the American Gemstone Association and served as a board member. She was a founding member of the International Colored Gemstone Association. A true gem lover, Sondra has marketed her treasures on the wholesale and retail markets.