Parts of the Stone

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HERE ARE THREE BASIC PARTS TO EVERY CUT DIAMOND:
  • The crown (top)
  • The girdle (around the middle)
  • The pavilion (the bottom).

CROWN The crown consists of a large flat area on top called the table, and a number of facets. As the diamond catches the light, the job of the crown is to split the light entering the diamond into white light, which gives the stone its brilliance, and colored light, which gives it fire, or dispersion.

GIRDLE The girdle is the thin, unpolished band around the widest part of the diamond. The function of the girdle is to protect the edge of the stone from chipping (even though diamond is the hardest natural substance on earth, it can be chipped!)

PAVILION The pavilion has the most important job, which is to reflect the light that passes through the crown back into your eyes. Think of it as a cone lined with mirrors. The light enters the diamond through the crown, splits into white and colored light, bounces off the facets of the pavilion back up through the crown, where you see it as sparkle! But to achieve the maximum sparkle — that magic combination of brilliance and fire — the diamond must be well cut and cut in the proper proportions.

THE SIZE OF THE TABLE, THE SYMMETRY OF THE FACETS, THE THICKNESS OF THE GIRDLE, AND THE ANGLE OF THE PAVILION MUST ALL WORK TOGETHER TO GIVE THE DIAMOND THE SPARKLE YOU WANT.

TABLE The size of the table, as a percentage of the crown, is important because it determines the amount of brilliance, or white light, the diamond will reflect. For example, if the table is 60% of the diameter of the crown, 60% of the light you see will be brilliance and 40% will be fire, or dispersion. Avoid a diamond with a table area of 65% or higher. It will give the diamond too much brilliance, and not enough fire–and the diamond will look fuzzy or foggy. (The only exception to this rule is emerald and rectangular cut diamonds that can have a 65% table. This includes all princess cuts, quadrillions and radiants.) HERE’S THE FORMULA: Table area 53-60% = GREAT! Table area 61-64% = GOOD! Table area 65%+ = except emerald and rectangular cuts.

FACETS The typical diamond is cut with 58 facets, 33 on the crown and 25 on the pavilion. On a well-proportioned stone, these facets will be uniform and symmetrical. If they are not, the diamond’s ability to refract and reflect light will suffer. Furthermore, a poorly-cut diamond just won’t look right to the eye. The sad fact is, 75% of all rounds and 88% of all other shapes on the market are poorly proportioned! Poorly proportioned stones are more profitable for the dealer, because they retain more of the weight of the rough or uncut diamond.

GIRDLE You don’t want a diamond with a girdle that’s too thin, or one that’s too thick — you want one that’s just right! The whole purpose of the girdle is to protect the edge of the stone from chipping. A girdle that’s too thin doesn’t give enough protection. A girdle that’s too thick does protect against chipping, but it doesn’t look so good. So you want a diamond with a medium girdle, neither too thin nor too thick. How do you tell? Look at the diamond from the side. If it looks like there’s a white chalk line around the middle of the stone, the girdle is too thick. If you don’t see any girdle at all with the naked eye, look at the same area of the stone with a 10X loupe. If you can’t see a girdle with the loupe, it’s too thin.

PAVILION The job of the pavilion is most important of all: to reflect light. The light enters the diamond through the table and the facets of the crown, passes through the diamond, and is reflected back by the facets of the pavilion. Here’s the important part: The angle of the pavilion for a round diamond must be between 40-41.5 degrees. 40.75 degrees is perfect. For marquise, pear, and ovals, the perfect angle perfect angle is 40 degrees, but an acceptable range is 39.25–40.75 degrees. For emerald and rectangular cuts, perfect is 45.05 and an acceptable range is 43.3-46.8 degrees. If the pavilion angle is not exactly right it will not reflect the light properly, and the diamond won’t have the sparkle it should. In a round diamond, there’s a dramatic loss of sparkle if the angle is even a tenth of a degree above 41.5 or below 40 degrees. In marquise, pear, or oval, maximum sparkle is achieved with a 40 degree pavilion angle, but the angle can be increased or decreased by as much as three-fourths of a degree with only a 10% loss of sparkle. Emerald and rectangular cut diamonds have the widest allowable variance of 1.75 degrees. Each extreme will also cause a 10 percent loss of sparkle.

CROWN ANGLE The angle of the crown is also important, but it doesn’t have to be quite as precise as the pavilion angle. The angle of the crown should be 32-35 degrees. If it’s smaller than 32 degrees, the diamond is what we call spread-cut. This makes the table area too large and the girdle too thin, and we already know what problems that causes. If the angle of the crown is above 35 degrees, it makes the diamond “top heavy.” This results in a smaller diameter, making the diamond look smaller than it really is. The last thing you want is a one-carat diamond that looks like a 3/4 carat!

CULET Finally, at the very bottom of the diamond–the base of the pavilion–there may be a small facet called the culet. If this facet is too large, when you look straight down through the table it will look like the diamond has a hole in the middle. Make sure the stone has no culet or a very small culet.

TWO OTHER IMPORTANT DIAMOND MEASUREMENTS Two other measurements to consider are: total depth and length-to-width ratio.

TOTAL DEPTH is a simple, straightforward measurement: take the height of the stone and divide it by the diameter of the stone. For a fancy stone, the diameter is measured at its widest part. The answer should be in the 56%-61% range. If it’s not, it means there’s something wrong with the crown angle and/or the pavilion angle, or the girdle thickness. (The only exception to this rule is emerald or rectangular cut diamonds that can have a total depth of 65%.)

THE LENGTH-TO-WIDTH RATIO is used to determine if a fancy-shaped diamond (anything other than round) is well-proportioned. For example, we don’t want to buy a marquise that is so skinny it looks like a banana, or one that’s so fat it looks like a football. Pleasing proportions aside, the length-to-width ratio also affects a phenomenon known as the bow-tie. Let me explain.

Fancy shapes are not symmetrical —  only a round is. And because fancy stones aren’t symmetrical, they all have a bow-tie — two triangular shadows in the middle of the diamond where light leaks out the bottom. If the length-to-width ratio is off, it will intensify the bow-tie in the stone! For a marquise diamond, the length should be no less than 1.75 times the width, and no more than 2 times the width. For pear shapes, the length should be no less than 1.5 times the width, and no more than 1.75 times the width. For emerald and oval shapes, the length should be no less than approximately 1.3 times the width, and no more than 1.75 times the width.

PROPORTIONS MADE EASY GIA has made it easier to determine if a diamond is well-proportioned by dividing all cut diamonds into four classes. Essentially, Class One and Class Two diamonds are well-proportioned; Class Three and Class Four diamonds are not. Class One diamonds are investment-quality stones, beautifully proportioned and priced to match. If your objective is to buy a beautiful diamond to wear, Class Two is fine.

FRED’S ADVICE: Don’t go below Class Two. And if the jeweler doesn’t know what the GIA classes are, move on!

PROPORTION AND PRICE A poorly proportioned diamond is worth as much as 40% less than a well proportioned stone. One reason for the difference in worth is that it takes a 3-carat rough, which is a diamond as it’s found in nature, to produce a well proportioned 1-carat cut stone. But it only takes a 2-carat rough to produce a poorly proportioned 1-carat stone. A poorly proportioned diamond will not sparkle nearly as much as a well proportioned diamond. If a diamond is poorly proportioned, only 35-40% of the light that enters it will reflect back up into your True Love’s eyes, while a well proportioned diamond will reflect close to 90% of the light. A woman wants a diamond to be “big, clean, white and sparkly,” and it won’t sparkle unless it’s well proportioned.

by Fred Cuellar, author of the best-selling book “How to Buy a Diamond.” More questions? Ask the Diamond Guy®