52 August 2006JEWELLERY NEWS ASIA

Gemstones: Origin Determination

The Limitations of Origin DeterminationIn this second instalment of a series of articles on the origin determination of coloured gemstones, theGubelin Gem Lab Ltd, in Lucerne, Switzerland, focuses on blue sapphires from Sri Lanka and from theIlakaka region of Madagascar. Gubelin shows here that it is not always possible to determine the originof gemstones with absolute certainty.

As we outlined in the first part ofthis series of articles (see JNA,

July 2006, page 66), the most impor-tant gemmological-mineralogical cri-teria used for the characterisation ofgemstones are:

• Inclusion features (cavity fillings,growth features, solid inclusions)

• Chemical fingerprinting (major,minor, trace elements)

• Spectral fingerprinting(UV-Vis-NIR range)

• Optical properties(refractive indices, birefrin-gence)

• IR characteristics• Luminescence behav-

iourThese gemstone proper-

ties are always – directly orindirectly – determined bythe genetic environment inwhich the stone was formed.The genetic environmentitself is mainly characterisedby the nature of the hostrock; “interactive events”between the host rock andits neighbouring rock units;temperature and pressureconditions; and the compo-sition and nature of solu-tions/liquids responsible forthe dissolution, transportand precipitation of thechemical componentsinvolved in crystal growth.

In the first part of thisseries, the case study,“Emeralds from the Colom-bian Cordillera Oriental andthe Santa Terezinha miningregion in Goias, Brazil,”illustrated the philosophy

and technical aspects of the geograph-ic origin concept. The key considera-tion was that emeralds originatingfrom these two areas were formed incompletely different host rocks. As alogical consequence, the Colombianand the Santa Terezinha emeraldsshow striking differences in their min-eralogical-gemmological features.

At this point, most readers will

understand that gemstones originat-ing in different geological environ-ments and localities will display dif-ferent properties, thus allowing theircharacterisation and separation. Butwhat of gemstones that were formedin similar genetic environments, eventhough their geographic regions arelocated great distances apart?

As a first approach to this ques-tion, it may be stated thatthe more similar the geneticenvironments of two gem-stones are, the more similartheir mineralogical-gemmo-logical properties will be. Todiscuss the consequences forgeographic origin determi-nation, we will use a casestudy of blue sapphires orig-inating in the ancient “gemisland,” Ceylon/Sri Lanka,and the new “treasureisland,” Madagascar. By farthe most important factorsin determining the origin ofblue sapphires are inclusionfeatures and chemical andspectral fingerprinting.Optical properties, IR char-acteristics and luminescencebehaviour are properties oflittle or no locality-specificimportance.

In Sri Lanka, there areseveral sapphire-producingareas of economic interest. Itis important to note thatdespite the fact that SriLankan sapphires originatein several secondary depositsin different parts of theisland (for example, Ratna-pura in the south-west and

Figure 1: Microphotograph of a blue sapphire from Sri Lanka,showing long, thin and oriented rutile needles, in association witharrow-twins of rutile

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Gemstones: Origin Determination

Elahera in the central region), the min-eralogical-gemmological properties ofSri Lankan blue, gem-quality stonessubmitted for origin determination arerelatively homogeneous. It is knowntoday that the source of most gem-quality sapphires in Sri Lanka aremetamorphic rocks of the gneiss/gran-ulite/charnockite types.

One of the most important gemsources today is without doubt theisland of Madagascar. This countryhas enormous potential for the min-ing of most gemstone species of cur-rent commercial interest. Sapphireand ruby deposits are located in dif-ferent regions of the country and arerelated to different types of hostrocks. Sapphires found in Madagas-car are characterised by high variabil-ity and complexity: numerous sap-phire deposits related to most differ-ent types of host rocks are dispersedover the entire island.

Examples are: 1) the basalt-relateddeposits at the northern tip of thecountry in the Antsiranana Province;2) the Andranondambo sapphireshosted by so-called skarn-type rocks

in the south-eastern part of the island;and 3) the huge secondary depositslocated in the Ilakaka and Andilame-na areas (south-west central north-east, respectively). It is assumed thatthe primary sources for the sapphiresfound in the secondary deposits aremetamorphic rocks (probably repre-senting several types similar to thesapphire host rocks in Sri Lanka).

The case study below comparesblue sapphires from Sri Lanka withtheir counterparts from the Ilakakamining region of Madagascar. It maybe assumed that the host rocks formost Sri Lankan blue sapphires andthe blue sapphires found in the Ilaka-ka region are of a similar nature.

Case study B: Blue sapphires from SriLanka and Madagascar (Ilakaka)

A basic difference between casestudy B and case study A (see JNA,July 2006, page 69) is that the emer-alds in case study A were found in situin their host rocks, whereas practical-ly all sapphires in Sri Lanka and ahuge part of those in Madagascaroriginate in so-called secondary

Figure 2: Microphotograph of a blue sapphire from Ilakaka (Madagascar), showingplanes and clouds of long, thick and iridescent rutile needles, often together with varioustypes of particles

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Gemstones: Origin Determination

deposits (generally in riversediments where they weredeposited after having beentransported from the placeswhere they were originallyformed). For gemstonesfound in secondarydeposits, the study of theirmineralogical-gemmologi-cal properties allows scien-tists to trace the stonesback to their host rocksand to draw conclusionsregarding the nature andtype of the host rocks.

(1) Inclusion featuresNumerous different

inclusion minerals found inSri Lankan sapphires aredescribed in the literature.Common in unheatedstones is rutile silk com-

posed of long fine threads[Figure 1]. Rutile may alsobe present in the form ofcoarser needles, “arrow-twins,” brownish-redprisms and roundedplatelets. Other guest min-erals in Sri Lankan bluesapphires are zircon (well-formed crystals, roundedgrains, often surrounded bya small system of fissures),spinel, apatite, mica,feldspar, pyrite/ pyrrhotite,uraninite, hematite andgraphite. Heat treatment –especially at high tempera-tures – will provoke thethermal alteration of mostmineral inclusions. In thiscase, they will typicallyshow a snowball-likeappearance. Often, the

altered mineral inclusionsare surrounded by atoll-like tension/healed fissures.

Rutile is the most com-mon inclusion mineral insapphires from the Ilakakaarea. Often, the rutile nee-dles occur in planes or inclouds [Figure 2] They tendto be long and thick andshow iridescent colours.Other inclusion mineralsidentified in Ilakaka sap-phires are apatite, feldspar,monazite, zircon, carbon-ate, graphite and mica.

As may be easily seen,most of the inclusion miner-als described above havebeen observed in both SriLankan and Ilakaka bluesapphires. It is true thatsome inclusions seem to be

more common in Ilakakasapphires (for example,monazite or zircon whenoccurring in groups of trans-parent rounded crystals)and some in Sri Lankan sap-phires (especially uraniniteand pyrite). There are alsodifferences in the “rutile sce-nario” in sapphires fromMadagascar and Sri Lanka.However, a clear separationof Sri Lankan and Ilakakasapphires based on the inter-nal association of inclu-sion minerals is not alwayspossible.

According to someauthors, cavities of varyingsizes and shapes (oftendeveloped as negative crys-tals) are considered veryreliable characteristics of

Figure 3: Microphotograph of a swarm of negative crystalstypically found in blue sapphires from Sri Lanka

August 2006JEWELLERY NEWS ASIA

Gemstones: Origin Determination

Sri Lankan sapphires[Figure 3]. These cavitiesusually contain fluid fill-ings. Also quite commonis the presence of smallblack-opaque plateletsdisplaying a metallic lus-tre (probably graphite).Cavities/negative crys-tals seem to be less fre-quent in Ilakaka sap-phires, but their pres-ence or absence is cer-tainly not a secure crite-rion for the separationof sapphires originatingin the two localities.

Further commoninclusion features in SriLankan blue sapphires are pro-nounced colour zones, dust bandsand abundant healed fissures display-ing a wide range of shapes, sizes andpatterns. Twinning is considereduncommon in Sri Lankan sapphires.The healed fissures of Ilakaka sap-phires also display a large variety ofpatterns, although these textures arefor the most part analogous or identi-cal to those observed in Sri Lankan

stones. Colour bands, colour zoning,dust bands and growth structures arethe dominant inclusion features inmany Ilakaka sapphires.

For such inclusions as twin planes,colour zones and dust bands, it maybe stated that even though some spe-cific features are more typical of SriLankan sapphires and others of Ilaka-ka sapphires, in many cases these donot provide enough evidence for a

clear separation betweenthe two.

However, there areinclusion features thatpoint strongly towards aMadagascar (Ilakaka)origin. These includeflake-like arrangements(often crystallographical-ly oriented [Figure 4])and tubes/fibres (mostlynot oriented; normallystraight and long; some-times wavy or bent; oftenshowing some epigeneticalteration [Figure 5]).

(2) Chemical fingerprinting

Generally, the following elementsare used for the chemical fingerprint-ing of sapphires: titanium (Ti), iron(Fe), gallium (Ga), and (with restrict-ed importance) chromium (Cr) andvanadium (V).

The iron content of Sri Lankanblue sapphires is considered low tomedium (normally below 0.30 weightpercent (wt %) Fe2O3). Titaniumshows a large variation, reaching val-ues up to ca. 0.07 wt % TiO2. Galli-um is generally low in Sri Lankanblue sapphires (< ca. 0.02 wt %Ga2O3). The elements chromiumand vanadium are quite common inSri Lankan sapphires, including theblue varieties. The upper concentra-tion limit for both elements is, in gen-eral, around 0.01 wt % Cr2O3respective V2O3.

The chemical fingerprinting ofblue sapphires from Ilakaka may bedescribed as follows. The iron con-tents show a considerable overlapwith Sri Lankan sapphires: ca. 0.05 to0.60 wt % Fe2O3, compared to ca.0.05 to 0.30 wt % Fe2O3 for SriLankan sapphires [Figure 6]. Thismeans higher iron contents are morecommon in Ilakaka sapphires. For theelement titanium, the concentrationrange in sapphires from both locali-

Figure 4: Microphotograph of flake-like particles, occasionally found inblue sapphires from Ilakaka (Madagascar)

Figure 5: Microphotograph of a blue sapphire from Ilakaka (Madagascar), showing long,bent, reflective tubes, partially filled with orangey staining

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Gemstones: Origin Determination

ties is practically identical: from 0.00to 0.07 wt % TiO2. The situation forgallium is almost the same: for bothdeposits, the upper concentrationlimit is normally ca. 0.02 wt %Ga2O3; only rarely are values up toca. 0.03 wt % measured.

The elements chromium andvanadium may also be present in bluesapphires from Ilakaka: V2O3 con-tents up to ca. 0.03 wt % and Cr2O3concentrations up to ca. 0.04 wt %have been reported.

Some ambiguity remains whenusing chemical fingerprints for theseparation of blue sapphires originat-ing in Sri Lanka from those originat-ing in Ilakaka. There is a substantialoverlap of the population fields inpractically all of the chemical correla-tion diagrams (compare, for example,the TiO2 vs. Fe2O3 diagram in Figure6). This means that chemical proper-ties alone are of limited help whentrying to separate blue sapphires fromSri Lanka and Madagascar.

(3) Spectral fingerprintingSri Lankan blue sapphires show

absorption spectra considered typicalfor corundums originating from a

metamorphic environment. They aregenerally dominated by a broadFe2+/Ti4+ charge transfer band withmaxima at 575 and 700 nm [Figure7]. The position of the UV-absorptionedge shows a large variation; Fe3+absorption groups are generally oflow or medium intensity. Absorptionbands related to Cr3+ and/or V3+ arenormally low; [Fe2+/Fe3+] chargetransfer bands are not developed.

Blue sapphires from Ilakaka dis-

play a large array of different UV-vis-NIR absorption spectra. There is notypical “Ilakaka spectrum,” butrather a collection of varying spectrathat may also be observed in sap-phires originating from other meta-morphic sources (such as Sri Lanka,Burma, East Africa or even Kashmir).The fact that the whole spectral vari-ation observed in Sri Lankan bluesapphires is covered by the differenttypes of Ilakaka specs is responsiblefor the limited value of spectral fin-gerprinting as a tool for the separa-tion of Madagascan and Sri Lankansapphires.

Case study A showed that theemeralds originating from the twogeographic localities in Colombia andBrazil are easily identifiable and sepa-rable because their mineralogical-gemmological properties are com-pletely different from each other. Thisstems from the fact that Colombianand Brazilian emeralds are formed indifferent geological environments(involving different host rocks, miner-alising solutions and pT-conditions,among others).

Ambiguity of originIn case study B, presented here, the

genetic environments in Sri Lanka

Figure 6: Correlation diagram of the concentration of iron and titanium oxides measuredin blue sapphires from Sri Lanka and Ilakaka (Madagascar), respectively. The outlinesindicate the representative distribution of the majority of analysed samples. The diagramshows the overlapping distribution fields of the two populations

Figure 7: Polarized (e and o beam) UV-Vis-NIR spectrum of a blue sapphire from SriLanka, with typical metamorphic features, i.e., dominated by the Fe2+/Ti4+ chargetransfer band with maxima at 575 and 700 nanometers (nm)

62 August 2006JEWELLERY NEWS ASIA

Gemstones: Origin Determination

and Ilakaka are supposedlyquite similar. This meansthe original host rocks(probably metamorphicrocks of the gneiss, gran-ulite, charnockite types), aswell as the growth condi-tions for the sapphires,should be widely analo-gous. As a logical conse-quence, it must be expectedthat the mineralogical-gem-mological properties of thesapphires originating fromthese environments are alsowidely overlapping. Thisassumption is confirmed bythe most important criteriaused for the characterisa-tion of blue sapphires: 1)internal features; 2) chemi-cal fingerprinting and 3)spectral fingerprinting.

It follows that the sepa-ration of blue sapphiresoriginating in Sri Lankafrom those originating inthe Ilakaka mining areas ofMadagascar may be verydifficult or sometimes evenimpossible due to theirmineralogical-gemmologi-cal properties being toosimilar.

Consequently, it is notpossible to determine thegeographic origin of allblue sapphires from thesetwo deposits with a satis-factory degree of certainty.Some Ilakaka and SriLankan sapphires showgemmological characteris-tics that occur in bothsources and thus do notalways allow for an unam-biguous determination ofthe origin. This possibleambiguity has conse-quences for gemmologicallaboratories doing origindetermination: if a gem-

stone does not show suffi-ciently clear evidence ofone source, the only profes-sional conclusion is to leavethe origin undetermined.

The case study present-ed here shows one of themost common examples ofwhen a reliable determina-tion of origin is not possi-ble. However, there are alsoother sources where gem-mological properties suchas inclusion features andchemical and spectral fin-gerprinting overlap to alarge extent and cause thesame difficulties when try-ing to determine the origin.

The situation raises anumber of questions. Howdoes this ambiguity affectthe feasibility and credibili-ty of origin determinationin gemstones? Should westop doing origin determi-nation altogether? Or arethere ways to manage thechallenge and continue toprovide the market and theconsumer with the infor-mation they are asking for:what is the country of ori-gin?

As shown here, somegemstones from certainsources lack a single diag-nostic inclusion or otherproperty that would allowfor the unambiguous deter-mination of their origin.However, the intelligentevaluation and interpreta-tion of the entirety ofobserved features, com-bined with additionaladvanced analysis, doesallow us to draw conclu-sions even in such difficultcases. We will discuss thisin more detail in the nextarticle in our series. JNA