De Beers Natural Versus Synthetic Diamond Verification Instruments

7/27/2019 De Beers Natural Versus Synthetic Diamond Verification Instruments

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DE BEERS ATURALERSUSBy Christopher M. elbourn, Martin Cooper, and Pau l M, pear

T w o nstrum ents have been developed atDe Beers DTC Research Centre,Maidenhead, to distinguish synthetic dia-mo nds from natural diamonds. TheDiainondSureTM nables the rapid ex am i-natio n of large nu mb ers of polished dia -mond s, both loose and set in jewehy.Automatically and with high sensitivity,this instrument detects th e presence o f the415n m optical absorption line, wh ich isfound in th e vast majority of natural dia-m o n d s but not in synthetic diamonds.Those stones in which this line is detectedare "passed" b y the instrum ent, and thosein wh ich i t is not detected are "referred orfurther tests." The Diam ondV iewT Rfro-duces a fluorescence image of the surface ofa polished d iamon d, from which thegrowth structure of t he stone ma y be deter-mined. On the basis of th is fluorescencepattern-which is quite diffe ren t or na tu-ral as compared to synthetic diamonds-the trained operator can positively ide nti fywhether a diam ond is natural or synthetic.

Riseseaacknowledgmentsat theend of article.OwnsGamotogy, Vol.32.No,3,pp. 156-169.6 996GOTofogtea/ nstitute ofAmerica

156 De Beers Verification Instru men ts

he subject of cuttable-quality synthetic diamondshas been receiving much attention in the gem trade

recently. Yellow to yellow-brown synthetic diamonds grownin Russia have been offered for sd e at a number of recentgem and jewelry trade shows (Shor and Weldon, 1996;Reinitz, 1996; Johnson and Koivula, 1996)) nd a small num-ber of synthetic diamonds have been submitted to gem grad-ing laboratories for identification reports (Fryer, 1987;Reinitz, 1996; Moses et al., 1993a and b; Emms, 1994;Kammerling et al., 1993, 1995; Kammerling and McClure,1995). Particular concern was expressed following recentannouncements of the production and planned marketing ofnear-colorless synthetic diamonds (Koivula et al., 1994;"Upfront," 1995).

Synthetic diamonds of cuttable size and quality, and thetechnology to produce them, are not new. In 1971, researchersat the General Electric Company published the results oftheir production of synthetic diamond crystals up to 6 mmaverage diameter by the high-pressure temperature-gradienttechnique using "belt1!-type presses (Wentorf, 1971; Strongand Chrenlzo, 1971; Strong and Wentorf, 1971). Theseincluded not only yellow-brown synthetic diamonds butalso reduced-nitrogen near-colorless crystals and boron-doped blue crysta ls (Crowningshield, 1971). In 1985,Sumitomo Electric Industries Ltd., in Japan, started market-ing their SumicrystalTMange of yellow-brown synthetic dia-monds; in 1993, they produced high-purity (i.e., near-color-less) synthetic diamond crystals fabricated into diamond''windows."De Beers Industrial Diamond Division (Pty)Ltd.has marketed its Monocrystal range of yellow-brown syn-thetic diamonds since 1987. None of these three manufac-turers has marketed synthetic diamonds for other thanindustrial or technical applications.

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-Figure 1. Like their predecessors i n m a n y other gem materials, cuttab le-quality synthetic d iamo nds pose apotential threat in th e diamon d marketplace. T o protect the integrity of na tural diam ond s should significantnu mb ers of syn thetic d iamo nds ever enter the trade, De Beers DTC Research Centre has designed and builttw o types of instruments-the DiamondSure and the Diamo ndview- that, together, can successfully iden tifyall synthetic dia mon ds produced by current synthesis equipme nt. This figure shows, bo ttom center and to theright, six De Beers experimental synthetic diamon ds: tw o yellow-brown samples weighing 1.04 and 1.56 ctand four near-colorless synthetics ranging from 0.41 to 0.91 ct. At th e top center and to t he left are six naturaldiamo nds, ranging from 1.10 ct t o 2.59 ct. It is su bstantially mor e difficult and costly to grow near-colorlesssynthetic d iam ond s than to grow th e more usual yellow-brown crystals. De Beers cuttable-quality syntheticdiam ond s are no t available commercially; they ha ve bee n produced solely for research and education.Natural dia mon ds courtesy of Louis P. Cvelbar and Vincen t Kong, V incent's fewehy, Los Angeles. Photo 0GIA and Tino Hamm id.

In 1990, researchers from Novosibirsk, Russia,published their work on the temperature-gradientgrowth of synthetic diamonds in relatively small-scale, two-stage multi-anvil presses laown as "split-sphere" or "BARS" systems (Pal'yanov et al., 1990).Since then, there have been a number of reports ofvarious groups within Russia intending to set upBARS presses for the purpose of synthesizing dia-mond. Usually, only one crystal is grown in a BARSpress at any one time, whereas many stones can begrown simultaneously in the larger "belt" presses.

Gemologists from GIA and other gemologicallaboratories have extensively examined syntheticdiamonds from each of the above-mentioned manu-

facturers. Gemological characteristics of these syn-thetics have been published in a series of articles inGems et)Gemology (Crowningshield, 1971; Koivulaand Fryer, 1984; Shigley et al., 1986, 1987, 1992;Rooney et al., 1993; Shigley et al., 1993a and b) andelsewhere (Sunagawa, 1995).The conclusion drawnfrom these studies is that all of the synthetic dia-monds examined to date can be positively identifiedby the use of standard gemological techniques.These results have been summarized in "A Chartfor the Separation of Natural and Synthetic Dia-monds," published by GIA (Shigley et al., 1995).

The problem facing the gem trade should syn-thetic diamonds become widespread is that, in gen-

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eral, most near-colorless diamonds are examined forgrading purposes only, not for identification.

It is to be expected that, in the main, syntheticdiamonds would be clearly identified and sold assuch by an honest trade working within nationallaws and regulations. Nevertheless, a small numberof synthetic diamonds have already entered thetrade without being declared as synthetic. De Beershas long regarded this potential problem as a seriousone. For the last 10 years, researchers at De BeersDTC Research Centre have been actively investigat-ing the characteristic features of synthetic diamonds(see, e.g., Burns et al., 1990; Rooney, 1992). Thiswork has been carried out in close collaborationwith the De Beers Industrial Diamond Division'sDiamond Research Laboratory in Johannesburg,South Africa, which has been developing hgh-pres-surelhigh-temperature diamond synthesis tech-niques for industrial applications for over 40 years.One aspect of this work has been the production of

Figure 2. The DiamondSue is based on the presenceor absence of the 415 nm h e n the stone being test-ed. Here it is shown with ts fiber-optic probemounted vertically for testing loose stones.On com-pletion of a test, which takes about 4 seconds, theliquid-crystal display on the front panel will give amessage of "PASS" or "REFER FOR FURTHERTESTS" (or sometimes "INSUFFICIENTLIGHT" ifthe stone is very dark or very strongly colored yellowor yellow-brown). Photo byM. .Crowder.

De Beers Verification Instruments

experimental cuttable-quality synthetic diamonds(figure I), with an extensive range of properties, bothfor research and for loan to the larger gemologicallaboratories throughout the world to give their staffmembers an opportunity to develop their own skillsand identification techniques. (Note that these syn-thetic diamonds are not for sale by De Beers. TheMonocrystal synthetic diamonds available commer-cially from De Beers Industrial Diamond Division(Pty]Ltd. are only sold in prepared forms and are notsuitable for cutting as gems.)A second aspect hasbeen the development of instruments that, shouldthe need arise, could be made available to helprapidly identify synthetic diamonds. Such instru-mentation would be important should near-color-less synthetic diamonds enter the market in sigmfi-cant numbers. If this were to happen, grading labora-tories and others in the trade would need to screensubstantial numbers of polished diamonds to eliini-nate the possibility of a synthetic diamond beingsold as a natural stone and thus damage consumerconfidence in gem diamonds.

Although such a circumstance would potential-ly have a profound impact on the conduct of thegem diamond trade, it is important to put the prob-lem into context. The high-pressure apparatusrequired to grow synthetic diamonds is expensive,as are the maintenance and running costs. In addi-tion, it is substantially more difficult and costly togrow near-colorless synthetic diamonds than it is togrow yellow-brown crystals. To reduce the amountof nitrogen (which gives rise to the yellow-browncolor) that is incorporated into the growing crystal,chemicals that preferentially bond to nitrogen areintroduced to the synthesis capsule. These chemi-cals, know11 as nitrogen "getters,"act as impuritieswhich have an adverse effect on the crystal growthprocess. To the best of our knowledge, the onlynear-colorless synthetic diamonds to appear on thegem market thus far were 100 Russian-grown crys-tals displayed at the May 1996 JCK Show in LasVegas. The largest of these weighed about 0.7 ct, buttwo-thirds of the crystals weighed 0.25 ct or less.Most of these were no t suit able for polishingbecause of inclusions, internal flaws, and distortedshapes.

Nevertheless, De Beers has considered it pim-dent to invest substantial resources to address thispotential problem and thus ensure that the trade isprepared for this eventuality. This article describestwo instruments, developed at De Beers DTCResearch Centre, that are capable of screening large

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numbers of diamonds and facili ta ti~.~he rapid andunambiguous identification of c-ynthetic diamonds.

Ideally, the trade would like to have a simpleinstrument that could positively identify a diamondas natural or synthetic with the-same ease as a ther-mal pen distinguishes between diamonds and non-diamond simulants such as cubic zirconia.Unfortunately, our research has led us to concludethat it is not feasible at this time to produce such anideal instrument, inasmuch as synthetic diamondsare still diamonds physically and chemically, andtheir distinguishing features are based on somewhatsubtle characteristics involving the presence orabsence of various forms of impurities and growthstructures. The ins truments developed at ourResearch Centre have been designed to be used in atwo-stage procedure. The first instrument, called theDiamondSvreT" allows the operator to screen largenumbers of stones rapidly. This instrument willsuccessfully detect all synthetic diamonds producedby current equipment (including experimental syn-thetics grown at the Diamond Research Laboratoryat extremes of conditions and with non-standardsolvent/catalysts). However, a small proportion ofnatural diamonds will also produce the sameresponse from the instrument. A second stage ofexamination is therefore required. This could be bystandard gemological examination. However, a sec-ond instrument has been developed, called theDiamondViewT~', hich enables a positive identifi-cation to be made quickly and easilyl. Certainaspects of the design of these instruments are propri-etary and so cannot be described in this article.However, we have endeavored to give sufficientinformation on their operation to show clearly howthey may be used to identify synthetic diamonds.TH E DIAMOND SURE^^SCREENING INSTRUME NTDescription.The Diamondsure (figure 2) has beendesigned for the rapid examination of large numbersof polished diamonds, whether loose or set in jewel-ry. It is 268 mm long by 195 mm deep by 107 mmhigh (10.6x 7.7 x 4.2 ins.), and it weighs 2.8 kg (6.2lbs.). Measurements are made by placing the tableof a polished diamond on the tip of a fiber-opticprobe, the diameter of which is 4 mm. For loosestones', the fiber-optic probe is mounted in a verticalposition, and a collar is placed around the end of .thel ~ h eiamondsure an d Diamondview instrum ents are coveredworldwide by granted or pending pa tent applications.


Figure 3. The Diamo ndsire probe can beremoved from its mounting t o test a diamond ina ring or other setting. Photo b y M. 1.Crowder.

probe so that stones can easily be positioned overthe probe tip (again, see figure 2). The instrumentcan be used on diamonds mounted in jewelry pro-vided that the table is sufficiently accessible for theprobe tip to lie flat against it (see figure3).When theprobe tip is in contact with the table of the diamondbeing examined, the operator presses the TEST but-ton on the front panel of the instrument or, altema-tively, presses the button mounted on the side ofthe probe. The time required for the instrument tocomplete a measurement is approximately 4 sec-onds. It is designed to work with diamonds in the0.05-10 ct range. This size range is determined bythe diameter of the fiber tip, because the instrumentresponds to the light that is retro-reflected by thecut diamond and re-enters the probe tip. Should theneed arise, fibers with larger or smaller diameterscould be manufactured to accommodate larger orsmaller stones. The instrument is powered by a uni-versal-input-voltage power supply, and so is suitablefor use in any country.

The instrument automatically measures theintensity of retro-reflected light in a small region ofthe spectrum centered on 415nm. Using proprietarysoftware, it compares the intensity data, as a func-tion of wavelength, to the 415nm optical absorptionline typically seen in natural diamond. The measure-ment is highly sensitive; values of 0.03 absorbanceunits at the peak of the 415 nm line, relative to abaseline through the absorption-line shoulders, are



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easily detected. We detected the 415 nm line in over95% of all natural diamonds tested by theDiamondSure instrument (see Test Samples andResults section, below), but not in any of the syn-thetic diamonds. If this feature is detected in astone, the instrument displays the message "PASS."If this feature is not detected, the message "REFERFOR FURTHER TESTS" is displayed. If very dark orvery strongly colored yellow or yellow-brown stonesare measured, the message "INSUFFICIENTLIGHT" may be displayed. With such stones, theoptical absorption in the wavelength range used bythe instrument is so strong that practically no lightis being returned to the detector. However, in thetests reported in detail in the Test Samples andResults section below, all the yellow-brown syn-thetic diamonds used-including the largest (2.53ct)sample-tested successfully. f the "INSUFFICIENTLIGHT" message is obtained with a particularlylarge stone, repositioning the stone on the probe willoften produce a valid measurement. If the probe tipdoes not lie flat against the table, the light detectedmay be composed mostly of light reflected from thetable without entering the diamond. In this case,the instrument would "fail safe" by "referring" thesample.Although the 415 nrn defect is not present in as-grown synthetic diamonds, it can be formed innitrogen-containing synthetics by very high-temper-ature heat treatment in a high-pressure press (Brozelet al., 1978). Temperatures in the region of 2350are required, together with a stabilizing pressure ofabout 85 lzbars to prevent graphitization. At theseextreme conditions, the lifetime of the expensivetungsten carbide press anvils becomes very short,the diamond surfaces are severely etched, and thelikelihood that the diamond will fracture is sigmfi-cant. Given the present technology, it would not,therefore, be commercially practical to heat-treatsynthetic diamonds to form sufficient 415 nmdefects.A small proportion of natural diamonds, lessthan 5% from our tests, do not exhibit the 415 nmfeature strongly enough to be detected by the Dia-mondsure. These include D-color and possiblysome E-color diamonds, as well as some browndiamonds. As for diamonds of "fancy" color, the 415nm line is absent from natural-color blue (type Db)diamonds, as well as from some fancy yellow andsome pink diamonds. When these stones are tested,the DiamondSure displays the message "REFERFOR FURTHER TESTS." It is important to recog-

nize that t h ~ :act that these stones were not"passed" by the 1i:strument does not necessarilymean that they are synthetic or in any way lessdesirable than stones that have been passed. Themessage simply means that additional testing isrequired for an identification to be made.Test Samples and Results. During the developmentof the DiamondSure, approximately 18,000 polishednatural diamonds were tested. In the final phase oftesting, which we report here, two instrumentsfrom an initial batch of 10 were each used to test atotal of 1,808 randomly chosen known natural dia-monds. Most of these 1,808 stones weighed between0.25 and 1.00 ct, although we included some assmall as 0.05 ct and some over 10 ct. The largeststone was 15.06 ct, and it tested successfully. Colorswere in the D to R range, as well as some brownsand some fancy yellows. In these particular tests, allexcept six stones were round brilliants; in an earlierexperiment, though, more than a hundred fancy-shaped stones tested successfully.The tests were carried out at the London officesof the De Beers Central Selling Organisation. Theinstruments were used by a number of operators. Ingeneral, a combination of daylight and fluorescentlighting was used, but no special care with respectto lighting conditions was necessary. The averagefigure for "referrals" for these 1,808 diamonds was4.3%.In a separate evaluation, we used a third Diarnond-Sure to test 20 D-color stones, of various shapes,ranging from 0.52 to 11.59 ct. Eight of the stoneswere passed, and 12 were referred. This indicatesthat, because of its sensitivity, the instrument candetect a very weak 415 nm line even in some D-color stones.The first two instruments were also tested on arange of De Beers experimental synthetic diamonds.A total of 98 samples were used: 23 in the yellow-brown range, 0.78-2.53 ct; 45 near-colorless,0.20-1.04 ct; 15 pale-to-vivid yellow, 0.19-0.63 ct;and 15 medium-to-vivid blue, 0.24-0.72 ct. (See fig-ure 1 for examples of the near-colorless and yellowDe Beers synthetic diamonds tested.) All of thesesynthetic diamonds were round brilliants except forone fancy yellow sample, which was an emeraldcut. Each was tested 10 times on each instrument.In addition, some yellow and near-colorless RussianBARS-grown synthetic diamonds were tested sever-al times on one of the instruments. In all cases, thesynthetic diamonds were "referred for further tests."

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THE DIAMOND VIEW^^ LUMINESCENCEIMAGING INSTRUMENTBackground: Growth Structure in Synthetic andNatural Diamonds. In the articles cited above on thegemological characteristics of synthetic diamonds,it was noted that the patterns of ultraviolet-excitedfluorescence exhibited by synthetic diamonds arequite distinctive and so can be used to positivelyidentify them. The Diamondview rapidly generatesthese fluorescence patt er ns ~w hich re produced bydifferential imp urity concentr ations betweengrowth sectors and growth bands-and providesclear images of them. With a little experience, it isrelatively easy to recognize patterns that are charac-teristic of natural or synthetic diamonds. With prac-tice, one can obtain and identify the fluorescenceimages of two or three diamonds per minute.The reason that fluorescence patterns can beused to identify synthetic diamonds is that the basicgrowth structure of synthetic diamonds is quite dis-tinct from that of all natural diamonds, and detailsof these growth structures can be inferred from thefluorescence pattern. Synthetic diamonds growessentially as cubo-octahedra. The degree of devel-opment of cube (100) or octahedral (111) facesdepends on a number of parameters, but mostnotably on the growth temperature. At relativelylow growth temperatures, cube growth predomi-nates; whereas at relatively high growth tempera-tures, the diamond morphology approaches that ofan octahedron, although small cube faces are stillpresent (Sunagawa, 1984; see figure 4). For syntheticdiamonds grown using pure nickel as thesolvent/catalyst, pure cubo-octahedra are produced.However, if other metals are used with or instead ofnickel, then minor faces of dodecahedra1 (110) andtrapezohedral (113) orientation also tend to be pre-sent (Kanda ct al., 1989; see figure 5a).In certain cir-cumstances (e.g., when cobalt is a constituent of thesolvent/catalyst, and getters have been used toreduce the nitrogen content), additional trapezohe-dral(115) aces may be present (Rooney, 1992; Bumset al., 1996).For large synthetic diamonds grown bythe temperature-gradient method, growth starts on aseed crystal of synthetic or natural diamond anddevelops outward and upward, as illustrated in fig-ure 5b. If the crystal shown in figure 5a were to besectioned along the planes A and B the growth pat-terns exposed by these planes would be as shown infigures 5c and d l respectively. (For a comprehensivebut easy-to-understand description of the numbers,or Miller indices, used to describe the orientation

I "" 1300 1400 1500 1600 1700 1800 1900TEMPERATURE PC)Figure 4. This schematic diagram shows t he depen-dence of synthe tic-diam ond morphology on growthtemperature (after Sunagawa, 1984). The Berman-Simon line separates the region in which diamond isthe therm odyn amica lly stable phase and graphite ismetas table (ab ove the line) from that w here graphiteis stable and diamond metastable (below the line).Diamond growth can only occ w to the right of thesolvent/catalyst melting line. The dashed linesapp roxim ately represent regions where simi111rmor-phologies are produced, in dicatin g tha t pressure isalso a factor in determining crystal shape.

and position of faces on a crystal, see J. Sinlzanlzas'Mineralogy, 1986, pp. 119-127.)Those regions of a crystal that have a commongrowth plane are referred to as growth sectors. Asthe crystal grows, different growth sectors tend totake up impurities in differing a mounts. Forinstance, nitrogen, the impurity responsible for theyellow to yellow-brown color in synthetic dia-monds, is generally incorporated at highest concen-trations in (1111 growth sectors, with the concentra-tion in 1100) sectors being about half that of (1111(Bums et al., 1990). (However, at low growth tem-peratures, the nitrogen concentration in (100) sec-tors exceeds tha t of (1111 [Satoh et al., 19901.1Nitrogen levels are substantially lower in the (113)growth sectors and very much lower in the (110)sectors. The polished slice of synthetic diamondshown in figure 6 was cut parallel to the (110)plane,with the seed crystal at the bottom and the (001)face at the top. The variation in nitrogen concentra-tion between growth sectors results in the zonationof the yellow color.Nickel and cobalt impurities can also be takenup by the growing crystal to form optically active

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P i p e 5. The idealized synthetic diamond crystal, seed-grown by the temperature-gradientmethod, exhibitsmajor octahedral {111) and cube J100)growth faces, and m inor dodecahedra1 I 101 and trapezoh edral {1131grow th faces (a).A vie w of the central section parallel to th e (110) dodecahedra1plane of this same crystal showsthe position of the seed at th e base of the crys tal, from whic h growth develops outward and upw ard (b) .The vari-ation in color saturation reflects the variation in nitrogen concentration between grow th sectors in yellow-brownsynthetic diamonds. The fluorescencepattern shown i n (c) s that of a section from this synthetic diamond crys-tal, parallel to the (001)cub e plane, indic ated by t he plane A in (a)and the lineA-A' in (b). n yellow-brown syn-thetic s, 11001 sectors t end to fluoresce green, { I 10) and 1113) tend to fluoresce blue. and { I 111 sectors are usu all ylargely inert. Th e fluorescence pattern s how n i n (d )is that of a (001)section indicated b y th e plane B in (a)andthe line B-B' in (b) .

defects, but they are incorporated exclusively in(1111 sectors (Collins et al., 1990; Lawson et al.,1996).In low-nitrogen synthetic diamonds, nickelgives rise to a green color; heat-treated cobalt-growndiamonds show a yellow fluorescence. Boron isanother impurity that is readily taken up by a grow-ing synthetic diamond. Blue, semi-conducting syn-thetic diamonds are produced by using chemicalgetters to reduce nitrogen levels and deliberatelyintroducing boron into the synthesis capsule. Boronconcentrations are highest for (111) sectors, nexthighest in (1101 sectors, and substantially lower inother sectors. Even when these impurities are not

present in concentrations high enough to influencethe color of the crystal, they still can cause fluores-cence behavior that varies between growth sectors.

For natural diamonds, the basic form of growthis octahedral. Small natural cubo-octahedral dia-monds have been found, but these are very rare (J .W. Harris, pers. comm., 1990). Dodecahedra1 andtrapezohedral flat-faced growth has never beenobserved in natural diamonds. Rounded dodecahe-dral diamonds are very common, but these shapesare formed by the dissolution of octahedral dia-monds (Moore and Lang, 1974). Figure 7a is aschematic diagram of a natural diamond in which

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Figure 6. This optical micrograpn 01 a slice cut paral-lel to the (110) dodecahedral plane from a De Beasyellow-brown synthetic diamond shows the greaterconcentration of nitrogen (and thus greater sat~rationof yellow) in the 1111)growth sectors than in the11001, 11131, or { I 101 growth sectors (again, refer to fig-ure 5b for a diagram of the differentgrowth stnzctuesin such a crystal a1 this orientation). The slice is 5.01mm across x 3.20 m m high x 0.71 mm thick.

the octahedral faces have undergone partial dissolu-tion so that rounded dodecahedral faces are hegin-ning to form. A schematic diagram of a sectionthrough a central cube plane of t h s idealized crystalis shown in figure 7b.

Dodecahedra1 faces that appear flat may be

found on "coated1! diamondsl but here the growth isfibrous and qui te distinct from flat-faced { l o]growth (Machada et al.! 1985).

A fonn of nonoctahedral growth that is relative-ly common in natural diamonds is so-called cuboidgrowth. The mean orientation of cuboid growth isapproximately along cube planes! but the growth ishummocky and dist inct from flat-faced cubegrowth. On the rare occasions that cuboid growth iswell developed compared to octahedral growth! dia-inonds with quite spectacular shapes are producedlas is the case with the "c~~ be s"ound in the Jwanengmine (Welboum et al.! 1989). t is not uncommonfor otherwise octahedrally grown damonds to haveexperienced a limited amount of cuboid growth! par-ticularly on re-entrant octahedral faces. This isshown schematically in figure 7b.

For most natural diamonds! the conditions inwhich they grew fluctuated over time! so differenttypes and levels of impurities were incorporated atdifferent stages of growth, This resulted in differ-ences in fluorescence behavior between growthbands within the crystal.

Uncut synthetic diamonds can be readily identi-fied by visual inspection because of their crystalmorphology and the remnants of the seed crystalpresent. However, these external features are lostwhen the stone is polished.

For many years, cathodolu~escenceopogra-phy has been used to image growth-dependent pat-

Figme 7. In this schematic diagram of (a ) the morphology of 1 typical natural diamond, the octahedral faces,decorated wit11 trigon etch pits, have undergone partial dissolution so that rounded dodecahedral faces arebeginning to form. The schematic diagram of the fluoresc~nce attern {ram a section through a central cubeplane of h s dealized crystal (b) hows concenLric rectangular baz~ds f octahedral grow~h nd regions where re-enuant features have been overgrown by cuboid growth.

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Figre 8. The Diamond-V ie w consists of a fluo-rescence imaging un it(left) in which he TVcamera i s locatedbetween tw o lamphousings (upper eft),with pecial stone hold -ers for loose (foregroundand figre 90) and ring-set (foregrou nd and fig-ure 9b) diamonds, anda specially configuredcomputer. Photo b y

terns in mineralsl including diamond (Woods andLang! 1975; Hanley et al.! 1977; Marshall! 1988;Ponahlol 1992). In cathodo lumine scence (CL)! a nelectron beaml rather than ultraviolet radiation! isu s e d t o e x c i t e l u m i n e s c e n c e . C o m m e r c i a l C Linstruments use a cold cathode dischuge tube oper-ating in a relatively low vacuuin to produce theelectron beam. Although CL is invaluable in thestudy of minerals! the fact tha t i t requires a vacuumcan be a disadvantage when large numbers of stonesmust be surveyed rapidly! as it may take severalminutes to pump down to the required pressure.Also! the surfaces of sam ples may becom e conta mi-nated by depo sits of proclucts from the p um p oil. Itwas to avoid these practical problems associatedwi th CL th at o ur Research Centre developed anultraviolet-excited fluorescence imaging technique.Description of the Diamondview. The Diamond-View consists of a fluorescen ce imag ing unit (60 cmh g h by 25 cin wide by 25 c m deep (24 in. x 10 in. x10 in.)lwhich weighs approximately 20 1% (44 lbs.j1and a specially configured comp uter (figure 8). Loosestones are mou nted between the jaws of a sto neholder that allows the stone being examined (from0.05 to approximately 1 0 ct) to be rotated abou t ahorizontal axis while it is being viewed (see figure9a). Rmg -mounted stones can also be examined! pro-vided that the to tal height of the ring is not too great(see figure 9b). Other simple jewelry mo unts canalso be accomm odated.The instrument i l l~lm inatesh e s ~ ~ r f a c ef a dia -


mond with intense ultraviolet light! specially fil-tered su ch tha t almost a ll of the light reaching thesample is of wav elengths shorter than 230 nm. T heenergy of thi s ultraviolet light is equal to or greatertha n th e in trinsic energy hand-gap of diamonds.This has two important consequences. First! radia-tion of this energy will ex cite fluorescence in practi-cally all types of diam ond irrespec tive of w he the rthey fluoresce to the standard long- and short-waveW radiation (365and 254 nm, respectively)routine-ly used by gemologists. Second! at wavelengthsshorter than 230 nm l all types of diam ond absorblight very strongly, T h s means that fl~~ore scencesgenerated very close to the surface of the & amo ndlso tha t a c lea r two-d imens iona l pa t t e rn can beobserved. The fluorescence emitted is viewed by asolid-state C CD (charge-coupled device) video cam -era that has been fitted with a variable-magnifica-tion objective lens. The camera has a built-in videopic t~l re torel and images can be integrated on th eCCD chip from 40 milliseconds up to 10 seconds!dependmg on th e intensity of the fluoresecence.To examine a stone! the operator inserts theloaded stone holder into the port a t the front of theunit. A n interloclcing safety mech anism eliminate sth e possibility of any ultraviolet light escaping fromthe instrum ent when the stone holder is out of theport. Th e stone is first illuininated wit h visible lightand the camera is focused onl say! th e table of thediamond. Th e stone is then illuminated with ultra-violet light and the fluorescence image is recorded.Th e instrum ent is controlled by anBM PC-compati-



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Figure 9.The loose-stone holder is inserted into the measzlrement port of the Diamondview (left),The gear mecha-nism allows the stone to be rotated abozzt0 horizontal axis while located within the instrument, Jor alignment andobservation of surface luorescence patterns characteristic of its internal g~owth tructures. The ring holder (right)canaccommodate a ring-set stone that has a total height no greater than 30 m m (1.2 in).Rings mounted in this holdercan be rotated about the axis of the holder and moved forward and backward along ihis axis. Photo byM. . Crowder.

ble computer running micros oft@ WindowsTh'3.1-compatible proprietary software. The computerhas a 12 MJ3z Pentium processorl 32 Mb of RATvI(random access me m ~ ry )~nd PC1 (peripheral com-ponent interconnect) video inp ~it nd graphics dis-play cards. The fl~~orescencemage is displayed on ahigh-resolutionf 1024 x 768 pixel! computer moni-tor. If additional views of the stone are required, thestone holder can be rotated! without removing itfrom the chamber! to bring other parts of the stone's

Figure 10. 7'his fluorescence image of a 0.3 ct near-colorless na t~ ral iamond shows concentric bands ofoc~ahedral rowth with a re-entrant eatzre belowthe center of the image and several regions of hum-mocky cuboid growth. The blue color is typical ofmost natural diamonds and results from so-calledband A emission together with some flzrorescencefrom the 415 nm system.

surface into view. Fl~~orescencemages that arerequired for future reference can be stored on thePC's hard drive. The number of images that can bestored is limited only by the size of the hard drive.In this modell the 800Mb drive could hold over 500images. Images can be archived using! for instance! atape drive or writable compact dislc. The displayscreen produced by the DiamondView software canbe seen in figure 8. The mouse-operated buttonsthat control the instnlinent are located beneath themain window! in which the current image is dis-played. This image may be compared with up to 16previously recorded images. These can be recalledon four pages! each of which has four "thumb-nailtfwindows! displayed on the right of the main win-dow. Tutorial files consisting of 16 'lthumb-nailllimages! complete with text notesf are provided inthe software to help the operator identify fluores-cence patterns. The user can also produce "cus-tomized!' t~itorialiles.Sample Images.The DiamondView was tested withthe same synthetic diamonds described above forthe Diamoi~dSure ests, together with about 150randomly chosen nat~iral iamonds. Following aresome examples of the images obtained. Figure 10shows the fluorescence image of a near-colorlessna t~~ra l.3 ct diamond mounted in an eight-clawring setting. The fluorescence in this sample rangesfrom bright blue to darli blue; it is typical for naturaldiamonds and results from so-called blue band Aemission together with some fluorescence from the415 nm system (see! e.g.! Clark et al., 1992). Thestone was polished such that the table is close to acube plane, and the striae visible in the image result

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from bands of octahedral growth intersecting thetable. Re-entrant features are evident in the lowerpart of the image) and cuboid growth horizons canbe seen in various placesl partic~~larlyoward theleft in the image. The concentric rectangular bands)the re-entrant feature below the center of theimage) and the hummoclzy cuboid growth bandsare all similar to those shown in idealized form infigurea.

Figure 12.In this yellow-brown plastically deformednatural diamond , approximately 0.1 ct, the fl~lores-cence imoge sho ws green H3 (503nm ) emiss ion/ram tw o sets o f parollel slip bonds. This typ e ofplastic deformation, covering th e entire stone, is no tu n w m m o n innatural diamonds, bu t it has not beenfound in synth e~ic iamonds.

166 De Beers Verification Instruments

Figure 11 . T h e f l ~ ~ o r e s -cence imoge of the ~o b le(left) of th is 1.5 c t na~u raldiam oi~d hows concen-tric bonds o f octahedralgrowth and a 11 L I ~er ofre-entrant features. Th epavilion of thi s sLone(right) shows some nar-row bright blue oc~ oh e-&a1 ban ds, with som ere-entrant features, in anotherw ise w ealzly fluo-rescing region.

The fluorescence iinage of a 1.5 ct near-colorlessnatural diamond is shown in figure 11 (left).Theban* is less pronounced in this stone than in theone shown in figure 10) but it is st ill apparent.However) the image of part of the pavihon of thisstone shows greater contrast) as is evident in figure11 (right).

The fluorescence image of an approximately 0.1ct yellow-brown natural diamond is shown in fig-ure 12. This diamond is plastically deformedl andthe green lines are produced by slip bands (planesalong which part of the crystal has undergone ashearing displacement) decorated by nitrogen-relat-ed H3 (503 nm) defects, Two sets of parallel slipbands may be seen. This type of plastic deforma-tion) which covers the entire stone) is not uncom-mon in natural diamonds but has not been found insynthetic hamonds,The Diamondview image of a 2.19 ct yellow-brown De Beers experimental synthetic diamond isshown in figure 13. From the syminetry of the pat-tern) it is clear that the table has been cut close to acube plane. This image may be compared with theschematic diagram shown in figure 5c. The central(001)sector is s~~rroundedy four other cube sec-tors) which fluoresce yellowish green) and by fourinert octahedral sectors. The yellowish green coloris due to the H3 (503 nin) system together withsome green band A emission (again)see Clarlz et al.)1992). Narrow) blue-emitting dodecahedra1 sectorslie between pairs of cube and pairs of octahedralsectors.

The fluorescence iinage of a 0.33 ct near-color-less De Beers experimental synthetic diamond isshown in figure 14 (left).Although the fluorescenceis blue) it is a less saturated) more grayish blue thanis typical of natural diamonds (again)see Shigley etal.) 1995).A brief examination of this image reveals

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Figure 13. Th e fluorescence ima ge of t he ta ble andsom e of the surrounding crown facets of this 2-19 ctyellow-brown De Beers experimental synthetic dia-mo nd show s yellowish green emission from the cen-tral(001) sector f l nd jo ~u ther cube sectors, Th e coloris due to the nitrogen-related H3 (503 nm ) systemtogether wit h som e green band A emission, The inertregions betwe en th e yellowish green cube sectors areoctahedral sectors. Narrow, blue-em itting dodecahe-&a1 sectors lie be twe en pairs of cu be sectors and pairsof octahedral sectors. Tra pez ohe drd 11 13) sectors hadnot developed significantly in this sample,a central (001)sector s~mounded y f o ~ romewhatbrighter octahedral sectors. Pairs of octahedral sec-tors are separated by narrow, less intensely emitting

dodecahedral sectors. The view of the pavilion ofthis stone (figure 14! center) shows the growth-sec-tor pattern even more clearly. A wealdy emitting(001)sector may be seen in the region of the culet.This is surro~~ndedy pale blue { I l l ) ectors lyingbetween nmow, less strongly emitting { lo] sectors.

The DiamondView has also been used to exain-ine a complete range of synth et ic diamonds!including both yellow and near-colorless RussianBARS stones. In all cases, the stones could be posi-tively identified as synthetic from their fluores-cence patterns.

Another feature of near-colorless and b l ~ ~ eyn-thetic diamoi~ds s that they tend to exhibit long-lived phosphorescence after excitation by ultravioletlight. Many natural diamonds do phosphoresce, butphosphorescence is relatively uncommon in near-colorless stones and is generally much weaker andfor a shorter period than in near-colorless and bluesynthetic diamonds. The DiamondView instrumenthas been designed to exploit this phenomenon inorder to assist further in the identification process.Phosphorescence images can be captured at timesfrom 0.1 to 10 seconds after the ultraviolet excita-tion has been switched off. An example of a phos-phorescence image from the 0.33 ct near-colorlesssynthetic diamond is shown in figure 14 (right).Theexposure time was 0.4 second, commencing after adelay of 0.1 second. Phosphorescence is strongestfrom octahedral growth sectors.

Figure 14, Th e fluorescence imag e of the crow n (left) o j this 0.33 ct near-colorless De Beers experim ental s yn -thetic diam ond shows a near-central(OO1) ector surrounded by four some what brighter octahedral sectors,which are separated b y narrow dodecahedra1 sectors. Th e blue w lo r is less saturated than is typical of naturaldiamonds. T he vi ew of the pavilion (center) shows a we akly em itting (001)sector in the region o f the C L ~ ~ Lur -rounded by pale blue octahedral sectors lying betw een narrow, less strongly em itting dodecahedra1 sectors.Aphosphorescence im age (right) , ecorded wi th an exposure tim e of 0.4 second an d a dela y of 0.1 second after theultraviolet excitation had been switched off, shows strongest vhosvhorescence from octahedral sectors. Strong,long-live d phosphorescence is a characteristic feature o less and blue synthetic diamo nds.

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We have loaned the GIA Gem Trade LaboratoryDiamondView and DiamondSure ins trumen ts,which they are evaluating for use as part of GIAGTL's standard diamond testing procedures. In thisevaluation, the Diamondsure is the first test for alldiamonds that the laboratory takes in (T . Moses,pers. comm., 1996).Using the DiamondView, GIAResearch in Carlsbad, California, recorded fluores-cence patterns on eight Russian and threeSumitorno Electric synthetic diamonds (all yellow).From these patterns, all of these diamonds werequickly and easily recognized as synthetic (J . E.Shigley, pers. comm., 1996).CONCLUSIONThe Diamondsure is a relatively inexpensive instru-ment capable of screening 10 to 15 stones perminute and automatically producing a "PASS" or"REFER FOR FURTHER TESTS" result. It is basedon the presence or absence of t he 415 nm line,which was found in more than 95% of natural dia-monds tested but has not been found in any syn-thetic diamonds. Because a small proportion of nat-ural diamonds would be referred by this instrument,additional testing may be required. TheDiamondView is a more complex and significantlymore expensive instrument. It enables the operatorto determine whether a diamond is natural or syn-thetic on the basis of a far-ultraviolet-excited fluo-rescence image. Synthetic diamonds are identifiedby their distinctive growth-sector structure, whereasnatural diamonds show either purely octahedralgrowth or a combination of octahedral and hum-mocky "cuboid" growth. Because only two or threestones can be examined per minute, and an operatormust interpret the fluorescence image, it would notbe practical to use the DiamondView alone forscreening large numbers of stones. It would there-

fore be appropriate for both instruments to be usedtogether or for operators of a DiamondSure to haveready access to a laboratory with a DiamondView.

At present, DiamondSure and DiamondViewinstruments are being loaned to a number of majorgem testing laboratories throughout the world. Bothinstruments have been designed so that they can bemanufactured in volume should near-colorless cut-table synthetic diamonds enter the gem market insignificant numbers. Although it has yet to beshown that this will be the case, these instrumentscould be made commercially available quickly,should a real need arise. The price of the instru-ments will depend very much on the numbers to beproduced, but it is estimated that a DiamondSureinstrument might cost in the region of a few thou-sand dollars, whereas the more complex Diamond-View might be 10times as much.

The development of these instruments ensuresthat synthetic diamonds of cuttable quality can beeasily identified. With such tools available to mem-bers of the gem trade, the existence of such synthet-ics should not be a cause of major concern.

- -~Aclznowledgments: Th e authors wish to tha nk all th eme mb er s of the staff at De Beers DTC lxesearch Centre,Maidenhead, wh o have been involved with the develop-m en t of the instru me nts described in this article.Particular recognition is d ue to P. S. Rose and G.M .Brown, for th e dev elop me nt of th e Dia~nondSure"~ins t rument , and t o T.M. ayman and R. M . Caddens,for the devel opm ent of th e DiamondViewrhrns t rument .S. f. Qu inn was responsible for m os t of th e instr um enttesting. The authors are also grateful to t he synthesisteam under Dr. R. C. Burns at th e Diamond ResearchLaboratory, fohannesburg, for th e sup ply of th e DeBeers experimental synthetic dia mo nds used to testthese instrumen ts .

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of nitrogen aggregates in diamond by high temperature-highpressure trea tments . Proceedings of the Royal Society ofLondon, A, Vol. 361, pp. 109-127.Bums R.C., Cvetkovic V., Dodge C.N., Evans, D.J.F.,Rooney M.-L.T., Spear P.M., Welbourn C.M. (1990)Growth-sector depen-dence of optical features in large synthetic diamonds. journalof Crystal Growth, Vol. 104, pp. 257-279.Burns R.C., Kessler S., Sibanda M., Welbourn C.M., Welch D.L.(1996)Large synthetic diamonds. In Advanced Materials '96:Proceedings of the 3rd NIRIM International Symposium on

Advanced Materials, National Institute for Research inInorgiinic Materials, Tsukuba, Japan, pp. 105-1 1 1.Clark C.D., Collins A.T., Woods G.S. (1992)Absorption andluminescence spectroscopy.In J. E. Field Ed. The Properties ofNatural an d Synthetic Diamond, Academic Press, London,pp. 35-79.Collins A.T., Kanda H., Burns R.C. (1990)The segregation ofnickel-related optical centres in the octahedral sectors of syn-thetic diamond. Philosophical MagazineB, Vol. 61, No. 5, pp.797-810.Crowningshield R, (1971) General Electric's cuttable synthetic

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diamonds. Gems a)Gemology, Vol. 13, No. 10, pp. 302-314.Ernms E.C. (19941Large gem-quality synthetic diamond identifiedby laboratory. Gem andJewellery News, Vol. 4, No. 1, p. 4.Fryer C.W. (1987)Gem trade lab notes: Synthetic diamond. Gemsa)Gemology, Vol. 23, No. 1, p. 44.Hadey P.L., Kiflawi I., Lang A.R. (1977)On topographically iden-tifiable sources of cathodoluminescc5nce n natural diamonds.Philosophical Transactions of the Royal Society of London,A, Vol. 284, No. 1324, pp. 329368.Johnson M.L., Koivula J.I., Eds. (1996)Gem news: Synthetic dia-monds are in the marketplace. Gems a)Gemolosy, Vol. 32,.No. 1, p. 52.Kammerline R.C.. Moses T., Fritsch E. 119931 Gem trade labnotes: Faceted yell ows ynt het ic diamond. Gems a)Gemology, Vol. 29, No. 4, p. 280.Kammerling R.C., McClure S.F. (19951 Gem trade lab notes:Synthetic diamond, treated-color red. Gems a)Gemology,Vol. 31, No. 1, pp. 53-54.Kammerling R.C., Reinitz I., Fritsch E. (19951 Gem trade labnotes: Synthetic diamond suite. Gems a)Gemology, Vol. 31,No. 2, pp. 122-123.Kanda H., Ohsawa T., Fukunaga 0 . (19891 Effect of solvent met-als upon the morphology of synthetic diamonds. Journal ofCrystal Growth, Vol. 94, pp. 115-124.Koivula J.I., Fryer C.W. (19841 Identifying gem-quality syntheticdiamonds:Anupdate. Gems a)Gemology, Vol. 20, No. 3, pp.146-158.Koivula J.I., Kamrnerling R.C., Fritsch E., Eds. (1994)Gem news:Near-colorless Russian synthetic diamond examined. Gemsa Gemology Vol. 30, No. 2, pp. 123-124.Lawson S.C., Kanda H., Watanabe K., Kiflawi I., Sato Y. (1996)Spectroscopic study of cobalt-related optical centers in syn-thetic diamond. J o u ml of Applied Physics, Vol. 79, No. 8, pp.1-10.Machada W.G., Moore M., Woods G.S. (1985)On the dodecahe-dial growth of coated diamonds. Journal of Crystal Growth,Vol. 71, pp. 718-727.Marshall D .J. (1988) Cathodolumi nescence of GeologicalMaterials, Unwin Hyman Ltd., London.Moore M., Lang A.R. (1974)On the origin of the rounded dodeca-hedral habit of natural diamond. lournal of Crystal Growth,.Vol. 26, pp. 133-139.

Moses T.. Kammerline R.C.. Fritsch E. f1993al Gem trade labnotes:Synthetic yellow di'amond crystal. ~ e h s9Gemology,Vol. 29, No. 3, p. 200.Moses T., Reinitz I., Fritsch E., Shigley J.E. (1993b)Two treated-color synthetic red diamonds seen in the trade. Gems a)Gemology Vol. 29, No. 3, pp. 182-190.Pal'yanov Yu.N., Malinovsky I.Yu., Borzdov Yu.M.,Khokhryakov A.F., Chepurov A.I., Godovikov A.A., SobolevN.V. (19901 Use of the "split sphere" apparatus for growinglarge diamond crystals without the use of a hydraulic press.Doklady Alzademii Naulz SSSR, Earth Science Section, Vol.315, No. 5, pp. 1221-1224.Ponahlo J. (19921 Cathodoluminescence [CL)and CL spectra ofDe Beers' experimental synthet ic diamonds. Journal ofGemmology, Vol. 23, No. 1, pp. 3-17.Reinitz I. (1996)Gem trade lab notes:A notable yellow syntheticdiamond from Russia. Gems a)Gemology, Vol. 32, No. 1, pp44-45.

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De Beers Natural Versus Synthetic Diamond Verification Instruments

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