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Diamond CutFacet proportions and names The modern round brilliant consists of 58 facets (or 57 if the culet is excluded); 33 on the crown (the top half above the middle or girdle of the stone) and 25 on the pavilion (the lower half below the girdle). In recent decades, most girdles are faceted. Many girdles have 32, 64, 80, or 96 facets; these facets are not counted in the total. While the facet count is standard, the actual proportions (crown height and angle, pavilion depth, etc.) are not universally agreed upon. One may speak of the American cut or the Scandinavian standard (Scan. D.N.), to give but two examples. Figures 1 and 2 show the facets of a round brilliant diamond. Figure 1 assumes that the "thick part of the girdle" is the same thickness at all 16 "thick parts". It does not consider the effects of indexed upper girdle facets. Figure 2 is adapted from Figure 37 of Marcel Tolkowsky's Diamond Design, which was originally published in 1919. Since 1919, the lower girdle facets have become longer. As a result, the pavillion main facets have become narrower. Fancy cuts Even with modern techniques, the cutting and polishing of a diamond crystal always results in a dramatic loss of weight; rarely is it less than 50%. The round brilliant cut is preferred when the crystal is an octahedron, as often two stones may be cut from one such crystal. Oddly shaped crystals such as macles are more likely to be cut in a fancy cut—that is, a cut other than the round brilliant—which the particular crystal shape lends itself to. Popular fancy cuts include the baguette (from the French, resembling a loaf of bread), marquise or navette ("little boat"), princess (square outline), heart, briolette (a form of the rose cut), and the pear or drop cuts. Generally speaking, these "fancy cuts" are not held to the same strict standards as Tolkowsky-derived round brilliants. Cuts are influenced heavily by fashion; baguettes—which accentuate a diamond's lustre and downplay its fire—were all the rage during the Art Deco period, whereas the princess cut—which accentuates a diamond's fire rather than its lustre—is currently gaining popularity. The princess cut is also popular amongst diamond cutters: of all the cuts, it wastes the least of the original crystal. In the 1970s, Bruce Harding developed another mathematical model for gem design. Since then, several groups have used computer models (e.g., MSU, OctoNus, GIA, and folds.net) and specialized scopes to design diamond cuts. During the 1990s Israeli interests, centralized in Ramat Gan, acquired about 20% of the diamond trade, buying diamonds from Russia and from mines in Africa not controlled by De Beers. De Beers now deals only in diamonds from their own mines. A major diamond cutting industry has grown up in the state of Gujarat, India where 90% of the world's diamonds (as measured by number of diamonds) are cut by a workforce of 800,000http://www.vibrantgujarat.com/sp-gems.html. Small diamonds previously not worth cutting are cut in India, opening up a new market segment for small diamonds. Some cuts are: - Round
- Radiant
- Pear
- Marquise
- Emerald
- Oval
- Heart
- Princess
The choice of cut is often decided by the original shape of the rough stone, location of the inclusions and flaws to be eliminated, the preservation of the weight, popularity of certain shapes amongst consumers and many other considerations. As far as the shape of the cut is concerned, it is very much a personal taste and preference. However, when jewelers judge the quality of a cut diamond, they often rate "Cut" as the most important of the "4-Cs." The key is not the shape, but how well the cutters executed that shape. The proportion, symmetry and quality of the polish are essential criteria of a good cut. Since the "brilliance" and "fire" of a diamond depends very much on the angle of the facets in relation to each other. A poorly cut diamond with facets cut only a few degrees from optimal will result in a stone that lacks the gem quality. For a round brilliant cut, there is a balance between "brilliance" and "fire". When a diamond is cut for too much "fire", it would look like a cubic zirconia which gives out much more "fire" than real diamond. A well executed round brilliant cut should reflect most light out from the tabletop and make the diamond appear white when viewed from the top. An inferior cut will produce a stone that appears dark at the center and in some extreme cases the ring settings may show through the top of the diamond as shadows. Sometimes the cutters compromise and accept lesser proportions and symmetry in order to avoid inclusions or to preserve the carat rating. Since the per-carat price of diamond is much higher when the stone is over one carat (200 mg), many one-carat diamonds are the result of compromising "Cut" for "Carat". Some jewelry experts advise consumers to buy a 0.99 carat diamond for its better price or buy a 1.10 carat diamond for its better cut. A 1.00 carat diamond is usually poorly cut stone. Cut grading The "Cut" of the "4-Cs" is the most difficult part for a consumer to choose in selecting a good diamond because a GIA certificate will not show the important measurements influencing cut (such as the pavillion angle and crown angle) and will not provide a subjective ranking of how good the cut was. The other 3-Cs can be ranked simply by the rating in each category. It requires a trained eye to see the quality of a good "cut". The relationship between the crown angle and the pavillion angle has the greatest effect on the look of the diamond. A slightly steep pavillion angle can be complemented by a shallower crown angle, and vice versa. Graphs showing this trade-off are available from folds.net, by pressing Go on the HCA web service, and in Bruce Harding's article on Faceting Limits. Other proportions also affect the look of the diamond: - The table ratio is very significant.
- The length of the lower girdle facets affects whether Hearts_and_arrows can be seen in the stone, under certain viewers.
- Indexing the upper girdle facets
- Most round brilliant diamonds have roughly the same girdle thickness at all 16 "thick parts".
- So-called "cheated" girdles have thicker girdles where the main facets touch the girdle than where adjacent upper girdle facets touch the girdle. These stones weigh more (for a given diameter, average girdle thickness, crown angle, pavillion angle, and table ratio), and have worse optical performance (their upper girdle facets appear dark in some lighting conditions).
- So-called "painted" girdles have thinner girdles where the main facets touch the girdle than where adjacent upper girdle facets touch the girdle. These stones (such as EightStar-brand diamonds) have less light leakage at the edge of the stone (for a given crown angle, pavillion angle, and table ratio).
Several groups have developed diamond cut grading standards. - The AGA standards may be the strictest. David Atlas (who developed the AGA standards) has suggested that they are overly strict.
- The HCA changed several times between 2001 and 2004. As of 2004, an HCA score below two represented an excellent cut. The HCA distinguishes between brilliant, Tolkowsky, and fiery cuts.
- The AGS standards will change in the first quarter of 2005 to better match Tolkowsky's model and Octonus' ray tracing results. The 2005 AGS standards will penalize stones with "cheated" girdles.
The distance from the viewer's eye to the diamond is important. The 2005 AGS cut standards are based on a distance of 25 centimeters (about 10 inches). The 2004 HCA cut standards are based on a distance of 40 centimeters (about 16 inches). History The history of diamond cutting can be traced to the late Middle Ages, before which time diamonds were enjoyed in their natural octahedral state. The first "improvements" on nature's design involved a polishing of the crystal faces—this was called the point cut. Later still, a little less than one half of the crystal would be sawn off, creating the table cut. Neither of these early cuts would reveal what diamond is prized for today; its strong dispersion or fire. At the time, diamond was valued chiefly for its brilliant lustre and superlative hardness; a table-cut diamond would appear black to the eye, as they do in paintings of the era. In 1375, there was a guild of diamond polishers at Nrnberg. In or around 1476 Lodewyk (Louis) van Berquem, a Flemish polisher of Bruges, introduced absolute symmetry in the disposition of facets. He cut stones in the shape known as pendeloque or briolette. About the middle of the sixteenth century, the rose or rosette was introduced. The brilliant cut was introduced in the middle of the seventeenth century. The first brilliants were known as Mazarins. They had 17 facets on the crown (upper half). They are called double-cut brilliants. Vincent Peruzzi, a Venetian polisher, increased the number of crown facets from 17 to 33 (triple-cut brilliants), thereby increasing very much the fire and brilliancy of the cut gem, which were already in the double-cut brilliant incomparably better than in the rose. Yet diamonds of that cut, when seen nowadays, seem exceedingly dull compared to modern-cut ones. Around 1900, the development of diamond saws and good jewellery lathes enabled the development of modern diamond cuts, chief among them the round brilliant cut. In 1919, Marcel Tolkowsky analyzed this cut. His calculations took both brilliance (the amount of white light reflected) and fire into consideration, creating a delicate balance between the two. His geometric calculations can be found in his book on Diamond Design. Related articles References - Tolkowsky, Marcel (1919). Diamond Design: A Study of the Reflection and Refraction of Light in a Diamond. London: E. & F.N. Spon, Ltd. (Web edition as edited by Jasper Paulsen, Seattle, 2001.)
- OctoNus Software has posted several diamond cut studies, by various authors. OctoNus, Moscow State University, Bruce Harding, and others have posted work there.
- Holloway, Garry (2000-2004). HCA: defining ideal cut diamonds is a detailed explanation of the "Holloway Cut Advisor". A web service that uses this software is available.
- Yantzer, Peter (March 2005). The effects of indexed upper half facets. (Retrieved March 19, 2005.)
* Various authors (2004). Pricescope discussion on "cheated" girdles. (Retrieved March 19, 2005.)
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