"Diamonds are the stuff of dreams, murder mysteries, poems and songs. They are the most popular of the "precious" gems. Their popularity, to a great extent, is the product of the kind of marketing that the Fortune 500 companies only hallucinate about in their most euphoric moments. It can be safely assumed that if the DeBeers cartel had not virtually monopolized the rough market and had not had such brilliant advertising, diamonds could easily have been just another gemstone. Unless you really study the history of this ambitious and mysterious company, and understand the full impact of the way people have been socialized to believe that diamonds are-at the least-a necessary, one-time purchase, you might consider their popularity as axiomatic as Newtonian physics. Although gems-quality diamonds are rare, they are more common than any other gem-quality colored stone. They only seem rarer because of the mystique DeBeers has created......for each handful of top-quality Burma ruby, Kashmir sapphire, large gem red spinel, or top-quality tsavorite, a whole driveway could be surfaced with fine quality diamonds." - David Marcum, in The Dow Jones-Irwin Guide to Fine Gems and Jewelry Diamond is the high pressure form of elemental carbon. The more common form of elemental carbon, graphite, differs from diamond by the arrangement of the carbon atoms and the strength of the carbon-carbon bonds. Carbon in diamond is bonded covalently (accounts for hardness); in graphite weak van der Waals bonds hold sheets of carbon atoms together. Diamonds are by far the most popular of all gems. David Marcum's comments about advertising not withstanding, the current popularity of diamond as a gem has its roots in two unique attributes: 1) an exceptional combination of physical and optical properties (hardness, luster, R.I., dispersion); 2) wide availability in both quantity and quality that have led to international standardization of marketing procedures and pricing. Diamonds have not always been held in such high esteem. Prior to the 18th century diamonds played a relatively insignificant role in the jewelry and gem trade. Part of the reason was that they are not particularly attractive in an uncut form; emeralds, rubies and sapphires, with their brilliant, vivid colors are far more attractive as polished rough. Until a 16-facet cut known today as the Mazarin cut was developed in the middle of the 17th century, there was no way to exploit a colorless diamonds' brilliance. Diamond cutting presents special problems that were not completely overcome until the early 1800's. The other reason was their relative scarcity; the only major sources of diamond until the huge South African discoveries in the late 1800's were the Galconda mines in India and later, rather limited, production from Brazil. Although the tradition of the engagement ring has a very long history, the prevalent use of diamond in such rings is a recent phenomenon largely created by DeBeers advertising. Only recently has such advertising begun to have a similar impact in such places as Japan and Germany, where such a "tradition" is only now being established.
High luster, extreme hardness, single refraction, and S.G. of 3.5 distinguish diamond from all natural and synthetic substitutes. On faceted stones the absence of scratches and sharp facet junctions can be diagnostic (c.f. glass or "paste").
Occurrence and Sources Gem diamond is found exclusively in the oldest parts of continents in an unusual brecciated, altered, ultrabasic igneous rock known as kimberlite, or in a closely allied rock, lamproite, and in sediments derived from their weathering. Kimberlite and lamproite are the end products of volcanic eruptions from sources in the earth's mantle. Both are found in restricted circular or oval-shaped areas at the surface which taper downward to subcylindrical "pipes" or conduits that extend to unknown depths. Laboratory research has shown that the pressures and reducing conditions required to crystallize gem diamonds and the inclusions they contain are only attained in the earth's mantle, at depths of 100 km or more, and perhaps at sites of large meteorite impacts. Isotopic dating of inclusions in diamonds has revealed that diamonds and the host rocks that contains them are usually not the same age, indicating that most diamonds do not crystallize from kimberlite or lamproite magma. The magma is thus only the transporting agent for conveying diamonds formed in other mantle rocks (eclogite and peridotite) to the surface. One of the most exciting, recent developments in geology has been the discovery of (micro)diamonds within eclogites of so-called "ultra high pressure" metamorphic terrains. These discoveries challenge the notion that volcanism is the sole means by which diamonds (and diamondiferous upper mantle rocks!) reach the surface. Though literally less than a handful of diamonds have ever been dated, most are remarkably ancient, a billion years or older! A few are considerably younger (Mesozoic). In contrast to the very old ages determined for most diamonds, eruption of some kimberlite and lamproite volcanoes occurred as recently as 50 million years ago. It has been suggested that the diamond pipes in the Kimberly area in South Africa (from where kimberlite takes its name), which erupted about 100 million years ago, erupted to a surface that is about 1500 meters above the present-day elevation of the area. It has also been estimated that the kimberlite that has since been eroded contained about 3 billion diamonds!
Geochemists have analyzed the isotopic composition of carbon (the ratio of carbon-13 to carbon-12) in diamonds and compared it to carbon in other minerals and rocks. Their work suggests diamonds are made of carbon that comes from two sources. Some diamonds have carbon that is identical to that in carbonate minerals (e.g. calcite in limestone) and hydrocarbons, suggesting they were derived from ocean floor or near-surface sediments that were recycled, through the process of subduction, into the mantle. Others contain carbon that is more like that expected if it were derived directly from parts of the mantle (peridotite) that still contain carbon from when the earth first formed, 4.5 billion years ago. Not all kimberlite and lamproite are diamond-bearing, and of those that do produce diamond, usually only a small fraction of the diamond is gem quality. Estimates vary and every mine is unique but quite commonly it takes 5 to 10 tons of kimberlite or lamproite ore to produce one carat of diamond rough. In most occurrences, this diamond rough will contain no more than 15% gem quality material, of which 70-80% are stones of less than one carat. It has been estimated that of the 50 million plus carats of diamonds being mined annually, only a few thousand carats can be cut into a colorless, flawless gem of a full carat or more. Because of this high waste to yield ratio, commercial diamond mining is usually undertaken at a grand scale, and is the most highly mechanized of all gem mining. Kimberlite diatremes are known on virtually every continent but most do not produce diamonds. Africa, particularly southern and western Africa (Zaire, now Democratic Republic of Congo, Botswana, Namibia, South Africa) contains the largest concentrations of diamond-bearing kimberlite. Other important sources are in northern Russia, Australia (source of much industrial-grade diamond) and Brazil. New discoveries in Canada that have recently come on-line may be a force in the future (a fascinating account of these discoveries can be found in an outstanding recent book). Lesser commercial producers are Venezuela, Borneo, and Guyana. The most productive mine in the world is the Argyle (AK-1) mine in Kimberley, northwest Australia. Only a very small percentage of the diamonds mined there are of gem quality and the diamonds are generally quite small (most finished stones less than 1 carat), but the mine is noted for having produced a large number of fine fancy pink diamonds. About 70% of world diamond production is controlled by the DeBeers cartel. In the US, the only diamond discoveries of any potential commercial significance are lamproite pipes at and near Crater of Diamonds State Park near Murfreesboro, Arkansas and kimberlite within the so-called State-Line Kimberlite District along the Colorado-Wyoming border. The Kelsey Lake mine in the State Line district was mined commercially from 1996-1998, during which time two notably large diamonds were discovered. One, a 28.18 carat crystal, yielded a 16.8 carat gemstone, the largest know cut diamond from the US. The mine is currently (11/99) not active, but was sold in the summer of 1999 to McKenzie Bay International, who is expected to resume operations. During the two years of operation, Colorado diamonds were reported to sell at a premium. Because of the extreme hardness of diamond, diamond cutting is a highly specialized process. Preforming usually involves cutting (not cleaving) with a diamond-impregnated saw, followed by further rough-shaping by a process known as bruting. Bruting is done by affixing the diamond to a dop stick, and shaping the stone with a diamond-tipped cutting tool (the brute). Faceting is done in the manner you were taught, with the exception that each facet is sometimes polished immediately after it is cut on a diamond-charged lap that contains separate concentric areas of differing coarseness. The principle cutting centers for diamond are in New York, Antwerp, and Tel Aviv. Most cutting of very small diamonds (melee) is done in India. U.S. cutters typically confine their work to gems over 1/8 carat, with the smaller gems being cut in Israel and Antwerp. Antwerp cutters are renowned for well-made fancy cuts. Smaller cutting centers exist in Amsterdam, London, Johannesburg, and San Juan, Puerto Rico. Although a wide variety of cuts have and continue to be used to facet diamond, by far the standard in this country is the Standard Brilliant or American Cut. Next in popularity are the Marquises and Pear-shapes. In the 50's and 60's step cuts were popular. Regardless of the cut, cut proportions are extremely important to the value of a diamond. This is one of the few aspects of stones that can be easily quantified by measurement, and there are established standards. Whether these standards do, in fact, yield the greatest brilliance is a subject of considerable debate. Recent research has demonstrated that it is likely that no single set of cut proportions yields better brilliance, dispersion and scintillation than all others and that there may, in fact, be several combinations that yield equivalent (or nearly so) appearance. Valid or not, certain parameters nevertheless have industry-wide recognition. For Standard Brilliants, the table percentage (diameter of table/diameter of girdle) should ideally be 52-58%. Commercial-quality jewelry diamonds are usually in the range of 53-66%; a stone bought with the intention of resale, or in stones over 1 carat (i.e. expensive ones), the table percentage should not exceed 59% (a "spread table"). For stones that are otherwise well cut, the depth percentage (distance from table to culet/diameter of girdle) should be about 60% for standard round brilliants. Well cut stones usually have proportions that range from 57% to 64%; more or less of these values will typically result in a lower cut grade. Stones that are less than 53% can be "fisheyed", while those greater than 65% will tend to show a dark spot (round cuts) or a darker "bowtie" (marquises, pears, hearts, ovals) centered on the culet. The depth percentage parameter includes the girdle thickness, which is less than desirable inasmuch as all girdles are not a uniform percentage of the diameter. Stones with a thick or very thin girdle, a shallow or overly steep crown, and/or a shallow or overly steep pavilion will generally result in a depth percentage that falls outside the preferred range, yielding a lower cut grade, as indicated below.
Abbreviations: Total Depth Percentage (TDP), Poor (P), Fair (F), Good (G), Very Good (VG), Excellent (Ex).
From GIA Newsletter, v.8, no. 16, 2006. The girdle should be thick enough to prevent chipping during mounting and wear and no thicker, and should be of uniform thickness around the stone. A guideline for maximum girdle thickness is 3% of the stones diameter. In no case should the thickness of girdle be less than 0.05 mm. Girdles with "naturals" (portions of the original rough that have not been faceted to conserve weight) that noticeably affect the roundness or symmetry of the stone, or extend into the pavilion or crown, should be avoided. A lack of polish on girdle facets does not detract from the value of a diamond, except when the girdle facets are thicker than normal. At least two other factors are important to cut, polish and symmetry. Polish is rated as poor to excellent, depending on the quality when examined at 10X magnification. Many diamonds do not have good symmetry, e.g. rounds may be lopsided, emerald cuts may have one wider end, marquises may have a crooked axis. The same rating is applied to symmetry as polish, and a good or better rating is important for top stones. There are, in addition, standard length-to-width ratios for pear, emerald, oval (all 1.5-1.75:1), marquise (1.75-2.25:1), and heart (1.1-1.25:1) cuts that can influence price (up to 10% of value) if not correct. Prices for well cut stones in these shapes may be as much as 20% more or less than the price of an equivalent round brilliant, depending on supply and current demand. Color in diamonds arises from trace amounts of nitrogen (yellow) and/or boron (blue) that substitute for carbon and act as either electron donors or acceptors with electron energy transition levels that are within the visible spectrum. Color can be induced or changed by irradiation and/or heating. Bombardment by high energy particles (electrons, neutrons, protons, gamma rays, alpha particles) is known to change pale yellow stones to fancy blue, green, brown, orange, very dark green, and yellow. Heating following irradiation can further modify the color. Treatment by all but gamma rays and neutrons colors only the outer few microns of a diamond's surface, producing an umbrella-like color zonation near the culet or an unevenness of color elsewhere. Heating changes the absorption spectra and can usually be detected with a spectroscope. Color change induced by bombardment of neutrons (nuclear reactor) is most difficult to detect; most stones turn green but show no color zoning or characteristic change in their absorption spectra. Color "enhancement" by any of these technique is not a mainstream practice in the gem trade. Coatings or backings have also been used to improve color. Coating applied to the pavilion girdle area can be used to mask a pale yellow color or give a blue or pink tint. Well applied coatings can be difficult to detect but sometimes give a grayish tint to the stone's color. Most recently (3/99), Pegasus Overseas
Limited (POL), a subsidiary of Lazare Kaplan International (LKI), has begun
to sell diamonds enhanced by a proprietary
process that converts certain brown to pale brown diamonds to colorless,
pale pink or pale blue.
The process, developed by General Electric and
now known to involve heating
at high pressures, may anneal
color centers responsible for the
color in these stones. Such gemstones are inscribed
"GE POL" on the
girdle, though the inscription may be removed
by recutting. Clarity in included diamonds has been improved by "lasering"; a laser is used to burn small holes to inclusions and acids are used to dissolve them. Such holes are, of course, permanent and even when filled can be detected with the aid of a loupe or microscope. Clarity has also been enhanced in diamonds with surface-reaching fractures by the use of fracture-filling compounds. Pricing and Valuation The well-known 4 C's - color, cut proportions, clarity, carat weight - comprise the basis for rating and pricing diamonds. Unlike colored stones, the wide availability of diamonds over a large range of quality has led to highly standardized grading practices. One of the most widely used rating schemes, and the one described below, is the G.I.A. (Gemological Institute of America) grading system. Though no method is exact, this system yields highly reproducible results, is easily understood, and requires little subjectivity. In purchasing a diamond of any size, you should ask what its grading parameters are. Jewelers should have this information (most purchase stones based on their grade) and be willing to share it with you. Many top stones may come with a "certificate", a grading report from a recognized laboratory (e.g. G.I.A., A.G.S., and E.G.L. in the US) that lists this and other information.
EXAMPLE CHARTS OF PERCENT DEDUCTIONS FOR DIAMOND CUTTING IMPERFECTIONS1
1 The stated "ideal" depth and table percentages are for Standard Brilliants only, as calculated and exhaustively documented by E. S. Love in an unpublished manuscript "A Manual of the Standard Round Brilliant - Optimizing the Proportions for Diamond and Colored Stones and Related Topics". With recent revisions and a more modern understanding of the combined affects of cut proportions, there is now widespread recognition that no one combination of proportions is better than all others and that there are, in fact, many of possible combinations that yield equally attractive gemstones. The system above is thus not a meaningful way of evaluating cut. |
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Notes Index
| Corundum | Beryl | Diamond | Pearl | Opal | Jade | Topaz | Tourmaline ©1998 |
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"I'm just
an old chunk 'o limestone, but I'm a gonna be a diamond some day.."
