Diamond cut
A diamond cut is a style or design guide used when shaping a diamond for polishing such as the brilliant cut. Cut refers to shape, and also the symmetry, proportioning and polish of a diamond. The cut of a diamond greatly affects a diamond's brilliance—a poorly-cut diamond is less luminous.
In order to best use a diamond gemstone's material properties, a number of different diamond cuts have been developed. A diamond cut constitutes a more or less symmetrical arrangement of facets, which together modify the shape and appearance of a diamond crystal. Diamond cutters must consider several factors, such as the shape and size of the crystal, when choosing a cut. The practical history of diamond cuts can be traced back to the Middle Ages, while their theoretical basis was not developed until the turn of the 20th century. The earliest diamond cutting techniques were simply to polish the natural shape of rough diamonds, often octahedral crystals; it wasn't until the 14th century that faceting, the process of cutting and polishing a gemstone to create multiple flat surfaces or facets, was first developed in Europe. Design, creation and innovation continue to the present day: new technology—notably laser cutting and computer-aided design—has enabled the development of cuts whose complexity, optical performance, and waste reduction were hitherto unthinkable.
The most popular of diamond cuts is the modern round brilliant, whose 57 facets arrangements and proportions have been perfected by both mathematical and empirical analysis. Also popular are the [|fancy cuts], which come in a variety of shapes, many of which were derived from the round brilliant. A diamond's cut is evaluated by trained graders, with higher grades given to stones whose symmetry and proportions most closely match the particular "ideal" used as a benchmark. The strictest standards are applied to the round brilliant; although its facet count is invariable, its proportions are not. Different countries base their cut grading on different ideals: one may speak of the American Standard or the Scandinavian Standard, to give but two examples.
History
Ancient India
The process of diamond cutting has been known in the Indian subcontinent as early as the sixth century AD. A sixth-century treatise Ratnapariksa, or "Appreciation of Gems", states that the best form in which to have the diamond is in its perfect natural octahedral crystal form, and not as a cut stone, indicating that diamond cutting was widespread practice. Al Beruni also describes the process of diamond grinding using lead plate in the 11th century AD. Agastimata, written before 10th century AD, states:A 12th- or early 13th-century diamond ring attributed to Muhammad Ghauri contains two diamonds whose crude octahedral natural states are maintained, but they are in limpid condition, exhibiting diamond polishing and shaping predating Europe, where the first diamond processing dates back to the mid-14th century AD. As of today, few diamonds with ancient Mughal-style faceting are known.
Europe
The history of diamond cuts in Europe can be traced to the late Middle Ages, before which time diamonds were employed in their natural octahedral state—anhedral diamonds simply were not used in jewelry. The first "improvements" on nature's design involved a simple polishing of the octahedral crystal faces to create even and unblemished facets, or to fashion the desired octahedral shape out of an otherwise unappealing piece of rough. This was called the point cut, and dates from the mid-14th century; by 1375 there was a guild of diamond polishers at Nuremberg. By the mid-15th century, the point cut began to be improved upon: the top of the octahedron would be polished or ground off, creating the table cut. The importance of a culet was also realized, and some table-cut stones may possess one. The addition of four corner facets created the old single 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 adamantine lustre and superlative hardness; a table-cut diamond would appear black to the eye, as they do in paintings of the era. For this reason, colored gemstones such as ruby and sapphire were far more popular in jewelry of the era.In or around 1476, Lodewyk van Bercken, a Flemish polisher of Bruges, introduced the technique of absolute symmetry in the disposition of facets using a device of his own invention, the scaif. He cut stones in the shape known as pendeloque or briolette; these were pear-shaped with triangular facets on both sides. About the middle of the 16th century, the rose or rosette was introduced in Antwerp: it also consisted of triangular facets arranged in a symmetrical radiating pattern, but with the bottom of the stone left flat—essentially a crown without a pavilion. Many large, famous Indian diamonds of old also feature a rose-like cut, although these Indian rose diamonds were most likely cut by European lapidaries. The rose cut continued to evolve, with its depth, number and arrangements of facets being tweaked.
The first brilliant cuts were introduced in the middle of the 17th century. Known as Mazarins, they had 17 facets on the crown. They are also called double-cut brilliants as they are seen as a step up from old single cuts. Vincent Peruzzi, a Venetian polisher, later increased the number of crown facets from 17 to 33, thereby significantly increasing the fire and brilliance of the cut gem, properties that in the Mazarin were already incomparably better than in the rose. Yet Peruzzi-cut diamonds, when seen nowadays, seem exceedingly dull compared to modern-cut brilliants. Because the practice of bruting had not yet been developed, these early brilliants were all rounded squares or rectangles in cross-section. Given the general name of cushion—what are known today as old mine cuts—these were common by the early 18th century.
Around 1860, American jeweler Henry Dutton Morse opened the first American diamond-cutting factory in Boston. Assuming that smaller but more beautiful gems would sell better, he went against the dogma of conserving diamond weight at all costs and scientifically studied refraction in diamonds, by around 1870 developing what was called the old European cut much later. This cut had a shallower pavilion, more rounded shape thanks to Morse's foreman Charles M. Field, who developed mechanical diamond-bruting machine to replace manual rounding, and different arrangement of facets. The old European cut was the forerunner of modern brilliants and was the most advanced in use during the 19th century and first two decades of the 20th century, prevailing on the market from about 1890 until about 1930. As compared with the modern round brilliant cut, it is inferior in brilliance but superior in fire.
Around the turn of the century, the development of motorized rotary saws for cutting diamonds, patented in 1901 by John H. G. Stuurman and in 1902 by Ernest G. H. Schenck, gave cutters creative freedom to separate small stones not detachable by cleaving as they wish and allowed them to waste less. These diamond saws and good jewelry lathes enabled the development of modern diamond cutting and diamond cuts, chief among them the round brilliant cut. In 1919, Marcel Tolkowsky analyzed this cut: his calculations took both brilliance and fire into consideration, creating a delicate balance between the two. Tolkowsky's calculations would serve as the basis for all future brilliant cut modifications and standards.
Tolkowsky's model of the "ideal" cut is not perfect. The original model served as a general guideline, and did not explore or account for several aspects of diamond cut:
Because every facet has the potential to change a light ray's plane of travel, every facet must be considered in any complete calculation of light paths.
Just as a two-dimensional slice of a diamond provides incomplete information about the three-dimensional nature of light behavior inside a diamond, this two-dimensional slice also provides incomplete information about light behavior outside the diamond. A diamond's panorama is three-dimensional. Although diamonds are highly symmetrical, light can enter a diamond from many directions and many angles. This factor further highlights the need to reevaluate Tolkowsky's results, and to recalculate the effects of a diamond's proportions on its appearance aspects....
Another important point to consider is that Tolkowsky did not follow the path of a ray that was reflected more than twice in the diamond. However, we now know that a diamond's appearance is composed of many light paths that reflect considerably more than two times within that diamond. Once again, we can see that Tolkowsky's predictions are helpful in explaining optimal diamond performance, but they are incomplete by today's technological standards.
Tolkowsky's guidelines, while revolutionary in their day, are not a definitive solution to the problem of finding the optimum proportions of a round brilliant cut diamond.
In the 1970s, Bruce Harding developed another mathematical model for gem design.
Since then, several groups have used computer models and specialized scopes to design diamond cuts.
The world's top diamond-cutting and -polishing center is India. It processes 11 out of 12 diamonds in jewelry worldwide. The sector employs 1.3 million people and accounts for 14% of India's $80 billion of annual exports. Its share in the world polished diamond market is 92% by pieces and 55% by value.
Theory
In its rough state, a diamond is fairly unremarkable in appearance. Most gem diamonds are recovered from secondary or alluvial deposits, and such diamonds have dull, battered external surfaces often covered by a gummy, opaque skin—a comparison to "lumps of washing soda" is apt. The act of polishing a diamond and creating flat facets in symmetrical arrangement brings out the diamond's hidden beauty in dramatic fashion.When designing a diamond cut, two primary factors are considered. Foremost is the refractive index of a diamond, which, at 2.417, is fairly high compared with that of most other gems. Diamond's RI is responsible for its brilliance—the amount of incident light reflected back to the viewer. Also important is a diamond's dispersive power—the ability of the material to split white light into its component spectral colors—which is also relatively high, at 0.044. The flashes of spectral colors—known as fire—are a function of this dispersion, but are, like brilliance, only apparent after cutting.
Brilliance can be divided into the definitions external brilliance and internal brilliance. The former is the light reflected from the surface of the stone—its luster. Diamond's adamantine luster is second only to metallic ; while it is directly related to RI, the quality of a finished gem's polish determines how well a diamond's luster is borne out.
Internal brilliance—the percentage of incident light reflected back to the viewer from the rear facets—relies on careful consideration of a cut's interfacial angles as they relate to diamond's RI. The goal is to attain total internal reflection by choosing the crown angle and pavilion angle such that the reflected light's angle of incidence falls outside diamond's critical angle, or minimum angle for TIR, of 24.4°. Two observations can be made: if the pavilion is too shallow, light meets the pavilion facets within the critical angle, and is refracted through the pavilion bottom into the air. If the pavilion is too deep, light is initially reflected outside the critical angle on one side of the pavilion, but meets the opposite side within the critical angle and is then refracted out the side of the stone.
The term scintillation brilliance is applied to the number and arrangement of light reflections from the internal facets; that is, the degree of "sparkle" seen when the stone or observer moves. Scintillation is dependent on the size, number, and symmetry of facets, as well as on quality of polish. Tiny stones appear milky if their scintillation is too great, whereas larger stones appear lifeless if their facets are too large or too few.
A diamond's fire is determined by the cut's crown height and crown angle, and the size and number of facets that compose it. The crown acts as a prism: light exiting the stone should meet the crown facets at as great an angle of incidence from the normal as possible in order to achieve the greatest fanning out or spread of spectral colors. The crown height is related to the crown angle, the crown facet size, and the table size : a happy medium is sought in a table that is not too small or too large.