Diamond Manufacturing

Diamond cutting: Considerably more talent and expertise is necessary for diamond cutting than that required of the colored stone factor. In order to successfully cut diamonds one must be able to look at a piece of rough and determine its crystallographic axes. This knowledge is then used in choosing the best cut to use, as well as in orienting and cutting the various facets, which must be ground in different directions on the lap, depending on their variable hardness.


The cutters must be adept at getting maximum yield while still retaining good brilliance. A 1-2% difference in yield would be small potatoes for most colored gem cutters, but in the case of a diamond, many dollars could hinge on it.


A "natural", a small unpolished area left on the girdle of a diamond, far from being a blemish, is a considered a sign of superior cutting--> meaning that the cutter has gotten the biggest stone possible from a particular piece of rough. In today's diamond cutting industry computerized angle analysis, and computer-assisted design (CAD), is often used to help cutters maximize yields.


In order to get the highest yields, most diamonds are cut with slight to moderate deviations from the "ideal" proportions, resulting in some loss of brilliance and/or dispersion. There are diamond cutting firms, though, which specialize in "ideal cut" diamonds. These stones are, on average, at least 10% more expensive per carat than "run of the mill" diamonds. In truth, only a well-trained eye can detect these subtle differences in proportions, dispersion and brilliance, but as with any product, there are those who are eager to have the "very best", and quite are willing to pay extra for it.


The equipment used in diamond cutting is heavy duty, reflecting the tremendous forces and long periods of time necessary to cut and polish this hardest of all materials. The lap, called a "scaife", is made of cast iron, and revolves at about 3000 rpm. Diamond "bort", in oil, is used for cutting and polishing. The stone is held by a "tang" which is a claw-like mechanism (no adhesive could withstand the intense heat generated by cutting).


The cutting and polishing process is really one of simple (but painstaking) abrasion as the tiny diamond crystals in the oil, being forced against the rough with great speed and pressure, successively wear away minute bits of diamond from the surface, gradually making it more smooth and uniform. Because the bort particles are randomly oriented, there are always at least some whose exposed crystal faces are harder than the surface to be polished (this is true as long as the diamond has been properly oriented for cutting to begin with--> its own hardest crystallographic axis cannot be directly parallel to any facet.)


The directions in which a diamond can be polished are called its "grain" and there are three basic types of rough (and many variations on those) each with a different pattern of grain directions. During the cutting process, as the different grain areas are brought to the scaife, the direction of the polishing must be altered. Twinning in diamonds can greatly complicate the cutting process, as there will be an overlapping of harder and softer axes. Only a few firms specialize in cutting twinned crystals.


Diamond cutting centers in New York, Bombay, Antwerp, and Tel Aviv produce the large majority of the world's diamond gems.

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Although there certainly are "master" cutters who can start with a piece of diamond rough, and go through all the steps to produce a finished gem; in the commercial world of diamond production, the process is usually divided into stages, each of which is accomplished by a specialist in that part of cutting.


1) Marking: The "marker" studies and then marks the rough to direct the removal of inclusions, and indicate how the piece should be cleaved or sawn. For large, extremely valuable pieces, this stage may take weeks or months.

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2) Cleaving and/or sawing: Although most of us can picture that tense moment when the "cleaver" swings his mallet and strikes the wedge that will separate a diamond along its cleavage plane, in reality, few diamonds are cleaved today. The average piece of rough is sawn (by diamond blade or laser) into suitably sized and shaped pieces by the "sawyer". Cleaving is still important with large rough however.

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3) Bruting: The job of the "bruter" is to create the face up outline of the gem: round, oval, marquis, etc. The time-honored technique for doing this involves using one diamond to grind another and is done mostly by eye using a lathe-like apparatus. When bruting is done with too much force, or too much heat is allowed to build up, tiny whisker-like feathers can be seen around the girdle of the stone. This is a blemish called a "bearded girdle".

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4 & 5) Blocking & Brillianteering: The "blocker's" job is to create the basic shape and proportions of the gem by cutting the table and culet as well as the crown and pavilion main facets. The "brillianteer" (the cutting superstar), traditionally puts in the stars, and the crown and pavilion bezel facets, each one of which may require subtle adjustments of angle, direction and size, depending on the grain pattern of the individual stone, its inclusions, and the desired cut.
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The Faceting Process

Faceting is the newest of the major lapidary crafts. Historically, we don't find the first faceted gems until the 14th century and faceted diamonds don't make an appearance until the 16th century. The earliest cuts were done by hand, and had just a few facets. An example of an early, but still occasionally used cut, is the rose cut, which was often chosen for early diamond jewelry. The rose cut has a flat bottom like a cabochon, with a series of facets rising to form a dome or apex.
Several hundred years ago the jamb peg faceting machine came into use, and faceting, as we recognize it today, began. In order to cut facets on a gem in an organized manner that results in a precise arrangement, three factors must be controlled:

 

1) The angle of the cut
2) The depth of the cut and
3) The radial placement of the cut. Although modern highly engineered faceting machines have replaced jamb pegs in much of the world, these older systems are still widely used, and they today have cut a substantial proportion of the gems in commerce.

 

 

Background Information on Faceting
Pavilion and Crown: In the faceted gem, the pavilion and crown have different functions. The crown acts as a window or lens to collect the light, which strikes it, and direct or focus it into the pavilion of the gem, whereas the pavilion must act as a mirror to reflect that light around the pavilion, and then back to our eyes through the crown. If the pavilion fails to do so, the gem lacks brilliance and is lifeless. Crown angles are much less crucial to the optical performance of a gem than are those of the pavilion, and can vary substantially from stone to stone without severely affecting a gem's brilliance. The crown and pavilion are cut in two separate sequences of operations. The gem is initially adhered to the "dop stick" until one side is finished, then removed, turned exactly 180 degrees, and attached to a new dop, to go through corresponding operations for the other side.

 

The Critical Angle: Each gem species, depending (with an inverse relationship) on its refractive index, has a pavilion faceting angle below which it loses brilliance.
Think for a moment of skipping flat stones on water. What controls whether the stone will skim and bounce along the surface, or go kerplunk into the depths?.......The angle at which it hits the water! So it is with light that enters a gem and strikes the pavilion facets. When that beam hits outside the critical angle it will be reflected to another facet and/or to the crown, but if it hits inside the critical angle it will not reflect, but pass right out through the side or bottom of the gem, not to return to our eye-->the gem loses brilliance.
In the graphics below we see two gems cut to the same proportions (pavilion main angles at 38 degrees) one is a diamond (RI = 2.42), the other is a fluorite (RI = 1.43). The critical angle for diamond is about 24 degrees that of fluorite is 44 degrees. At 38 degrees on the pavilion facets much of the light hitting the fluorite is lost, whereas almost all that which hits the diamond is reflected. The diamond would appear bright and the fluorite lifeless, especially in the center: we would say it has a "window". If, instead, we were to cut the fluorite to a pavilion angle of 45 degrees or above, we would then eliminate the window and it would be brilliant, and conversely we would get a lifeless diamond if we were to cut its pavilion at 20 degrees or below.

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In the first picture below you can see two similar looking gems (each is light yellow and rectangular). The golden beryl gem on the left was cut with its pavilion facets above its critical angle, and it appears brilliant, the yellow spodumene on the right was cut with the pavilion facets below its critical angle and has a "window". We call it a window because the light passes right through it, like window glass, so that you can easily read the printing underneath. The second set of pictures shows a top and bottom view of a badly windowed topaz. You can see how shallow (low angle) the pavilion is. In order for this gem to be fully brilliant, the necessary recutting would reduce its face up diameter and carat weight substantially.

Yield vs. Brilliance, Clarity and Color: Faceting is a series of compromises. The yield, that is the carat weight, of the finished stone versus the carat weight of the rough, can be as high as 40 - 50% or as low as 1-2% depending on a mix of the attributes of the rough, and of decisions that are deliberately made by the factor.
For example:
1) The shallower the pavilion angles, the greater the yield (but the less the brilliance).
2) Included rough can be oriented (with loss of yield) to eliminate or minimize the appearance of inclusions.
3) Pleochroic stones will give different colors and different yields depending on how the stone is oriented for cutting.
4) Rough that happens to be somewhat "gem shaped" yields more than thin and flat, or highly asymmetrical rough.



Given a moderately well shaped, clean piece of rough, which is cut to correct pavilion angles, the average yield is about 20%. To put it another way: start with gem rough = 5 ct, end up with finished gem = 1 ct.

 

Native cut gems: These are generally cut by eye, usually in Asia, Africa or S. America, on a lap, or more often a jamb peg or similar machine. The cuts are typically oval to cushion shaped with windows, low crowns and bellied pavilions. The "make" (proportions and finish) is "inferior" in that the table sizes, crown heights, pavilion depths; facet meets and degree of polish are not to custom standards.
Such cuts give high yield from gem rough, and due to the increased volume/mass of the pavilion tend to deepen and emphasize color. For these reasons the majority of the high value colored gem rough (ruby, sapphire, emerald, Imperial topaz, etc) is still cut this way-->even though the cutters could make smaller, brighter stones with the equipment they have. Frequently native cutters are paid by yield, another factor which perpetuates the native style of cutting.

 

The "faceting cookbook": Although some factors do freehand cutting or design and execute their own special cuts, most follow diagrams that indicate the angles, number and cutting sequences for the facets. (Due to critical angle differences, the directions for a round brilliant topaz would have slightly different angles than those for round brilliant quartz).
In the graphic below "B" = bezel facets, "M" = main facets, "S" = star facets, and "T" = table. ID indicates the "12 o'clock position" for the index gear. (On the left is the pavilion-cutting diagram, and on the right the crown cutting diagram).

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For example, the first instruction in such a facetor's "cookbook" might say (for quartz) "Cut 8 equidistant 42 degree facets as the pavilion mains, so that they meet to form a culet". This would be followed by something like: "At index settings on either side of the main facets, cut 16 bezel facets at 44 degrees". The directions would continue through the entire cutting process.



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