Magazine • Testing jewelry, precious metals and gemstones


Testing jewelry, precious metals and gemstones

by Gemmologisches Labor Berlin

Which laboratory tests should importers of jewelry be aware of, and how can a testing laboratory determine the origin of a gemstone? testxchange spoke with Andreas Stratmann, Managing Director of Gemmologisches Labor Berlin.

testxchange: Mr. Stratmann, as a gemologist and goldsmith, you test both gemstones and precious metals in your testing laboratory. What are typical testing requests you receive from customers on a daily basis?

Andreas Stratmann: We are primarily concerned with gemstone determinations, diamond gradings, the determination of precious metals and their alloys, as well as the valuation of jewelry with gemstones and diamonds. Frequently, my services are also used to verify dealer information or existing certificates, for example, when it comes to particularly valuable jewelry or larger gemstones and diamonds.

Since allergic reactions have also increased significantly, as well as the tarnishing (oxidation) of jewelry even with a fairly high fineness, we often also test for nickel and other elements that trigger allergies. Corresponding analyses provide clarity here.

Moreover, in the event of a legal dispute, the court may need proof that the jewelry is not valuable gold jewelry, but merely gold-plated costume jewelry. For this purpose, we then carry out a layer thickness measurement using X-ray fluorescence analysis.

In the case of jewelry and gemstones, e.g. TV shows often suggest that it is possible to look at a stone with a magnifying glass and guess the type of gemstone, its quality and condition, as well as any treatments that may have been applied, and even be able to name a value. Such an approach is not only dubious, but also raises false expectations in the consumer. For a systematic gemstone determination, gemological devices, a gemstone microscope and modern spectrometers are necessary.

For the gemstone ruby alone, there are several different synthetic manufacturing processes, some of which are combined with each other to form hybrids and, following manufacture, are also subjected to heat diffusion treatment, for example, in which foreign substances such as beryl are introduced into the stone. The purpose of this is to make a less experienced examiner or expert think that he is dealing with a treated ruby, but not with a synthesis. Ruby and sapphire belong to the gem group corundum, and synthetically produced corundum unfortunately has the same gemological values as naturally formed corundum. The traditional color spectroscope is of little diagnostic value, leaving the appraiser to rely on microscopy. However, gemstone microscopy requires experience and time for examination, and sometimes ambiguities still remain. Modern, non-destructive examination methods are then necessary to clear these up.

Again and again, the principal questions are like: Is my gemstone natural, treated, or even just synthetic? Do I have a natural diamond, is it HPHT-treated, was it artificially produced or is it just a CVD-vaporized moissanite (i.e. another stone that was merely vaporized with diamond). The latter can no longer be determined with the usual diamond moissanite testing equipment. Today, modern examination methods are also essential for this.

Not only the private end user, but also experienced colleagues approach us with these questions. Our spectral and X-ray fluorescence analyses are also commissioned by expert colleagues, as they are often used as evidence in expert reports. Through the analyses, scientific facts are substantiated and an expert opinion is then no longer merely the expression of an expert's opinion.

Some questions can also be answered without much effort with just a few testing procedures, such as: Is my light blue stone a blue topaz, an aquamarine, a synthetic light blue spinel or just glass? For this, a thermal resistance measurement and examination with a polariscope is sufficient at first. If it should be an aquamarine, only then further examinations would be necessary. By such preliminary examinations one can avoid producing unnecessary costs.

One popular service are our mini-certificates for gemstones, which contain a picture of the stone and, in addition to the most important values as a fingerprint of the stone, a picture of the Raman spectrum. As an extension, the preparation as an investment stone is also often ordered. The stone is sealed in a gemstone box, in which the mini-certificate is inserted. The stone and the certificate are then sealed again in plastic for safekeeping.

Since compliance with certain pollutant limits has become a legal requirement not only in electrical appliances, and since one of the major sales platforms on the Internet also requires sellers to provide proof of this, RoHS pollutant testing has also become an important topic.

In addition to RoHS certification, small and medium-sized lamp manufacturers and importers also have their lamps tested if, for example, it needs to be proven that a lamp used for curing UV adhesives or for disinfecting has precisely certain wavelengths in the ultraviolet range.

Andreas Stratmann
Gemologist, certified diamond appraiser, expert, book author, master goldsmith: Andreas Stratmann

testxchange: The testxchange platform repeatedly receives inquiries from retailers and importers who have imported fashion jewelry from abroad and are uncertain about the alloys used and conformity with EU market access requirements. What information and documents should importers request from the manufacturer in such cases?

Andreas Stratmann: The CE certificate does not necessarily have to be issued by the manufacturer. Apparently, it is little known that an importer can declare himself that his articles are compliant after he has fulfilled the corresponding respective requirements. From experience, I can say that some companies first import a few samples, which we then test for harmful substances. After passing this test, a RoHS certificate is then issued, which is then attached to the documentation together with the importer's declaration of conformity before shipment and made available to the carrier. This procedure is important because, for cost reasons, at least one Euro container is filled, the return of which is de facto impossible. However, we as a testing laboratory are the wrong contact for questions in this regard. There are consulting companies and customs agencies for this. At this point I would also like to ask for your understanding that I am obliged to neutrality and independence both as an expert and as a testing laboratory. For this reason, I do not purchase any items evaluated by me and cannot make any recommendations.

In Germany, the fineness of gold is given in parts per thousand. 750/000 Au therefore means that 750 g of 1000 g of the alloy is pure gold. Hallmarks (stamps) in carats, such as 18K, are not permitted in Germany under the Reichsstempelgesetz (Imperial Stamp Act), which is still in force. In the case of gold plating, no additional indication of fineness may be given because of the possibility of misleading the consumer.

In the case of jewelry and costume jewelry, it must be proven with regard to the metals and precious metals that the limits for harmful substances are complied with. For this purpose, we carry out the RoHS analyses. In the case of costume jewelry, a piece of the corresponding batch should always be allowed to be destroyed for analysis.

In the case of minerals and gemstones, it should be noted that alleged detected contaminants may also be elements that are natural components of the chemical composition of the stone. In the case of emerald, for example, chromium and vanadium are the color-giving elements, and also a ruby is only a ruby if the color is caused by chromium, otherwise it would be a red sapphire.

Chromium and vanadium, as natural elements in an emerald crystal, naturally do not represent a hazard and must be taken into account accordingly in a pollutant analysis. One cannot simply label the elements detected by the device as hazardous substances here.

Toxic trace elements such as cadmium are not uncommon in precious metals. Gold alloys are hard soldered in jewelry manufacturing. The gold solders have the same fineness as the parts to be joined. The gold solder has a slightly lower melting point than the alloy to be soldered. Lowering the melting point of the gold solder has been achieved by adding cadmium, which has been banned in the EU for several years.

A check of existing gilding is also always useful and important. With XRF, we can perform a layer thickness measurement. The result should then essentially match the manufacturer's specifications.

On older costume jewelry you can sometimes find hallmarks (stamps) such as: "American", "Union" and "Charnier". These names denote predefined high layer thicknesses. Because of the high price of gold, you can usually find only very thin galvanic coatings, which can not even prevent oxidation, tarnishing of the piece.

A RoHS certificate is also useful for marketing strategy reasons, to show the customer that the products are harmless to health. Further information on import requirements, such as sanctioned countries from which imports are not permitted, can be obtained from the customs authorities, for which customs agencies also offer their services, and law firms should be consulted for any legal advice that may be required. As a basic requirement for the import, a value added tax identification number (VAT ID) is needed. In addition, an EORI number is required for smooth customs clearance. Before importing, the corresponding customs tariff numbers must be researched for the respective articles. Furthermore, it is of course necessary to find out whether there are import restrictions or bans for the respective country and/or for the item to be imported, whereby the export regulations of the country from which the import is to be made must also be observed.

testxchange: And if the available documents are not sufficient, what laboratory tests would you recommend for imported jewelry in general, e.g. regarding nickel, other heavy metals and RoHS?

Andreas Stratmann: It's good that you bring up the subject of nickel. Here we first have to ask ourselves why nickel and gold are actually alloyed. Only gold and copper are yellow and reddish. All other metals and precious metals are gray. Now we ask the crazy question of how to get the beautiful yellow color away from the gold to alloy white gold. For this, the goldsmith uses a sinfully expensive alloy of gold, palladium and some silver and copper, which he buys as so-called semi-finished products (white gold sheet, wire, tube, etc.) from refiners. No white gold jewelry can be produced in this way for the ordinary consumer. Therefore, the industry switches to master alloys that are advertised as nickel-free, but still contain nickel. Before galvanic coating with rhodium, nickel white gold is not really white, but slightly grayish-yellowish, which is unpleasant for the customer when the rhodium coating on his industrially produced nickel white gold ring has worn off after some time. Rhodium plating brings us to the next topic. Rhodium is one of the 8 precious metals and next to osmium the most expensive precious metal. A small bottle of rhodium liquid for the galvanic top coating costs several thousand euros.

Moreover, the electroplating bath is very sensitive and can be destroyed by the smallest operating errors. An electroplated protective layer of rhodium is shunned today for cost reasons and many manufacturers switch to other metals or simply do not coat at all.

Unfortunately, there is no general ban on nickel, but only a regulation that states that only a certain defined amount of nickel may be released from a piece of jewelry onto the skin. There is a test procedure for this, but it is not non-destructive and also not comprehensible for the end user. A limit value for the element nickel is not provided for in the RoHS pollutant test.

In older pieces of jewelry made of nickel white gold, experience has shown that the nickel content was often close to 10%, which is of course greatly excessive. We can rhodiumize in such cases until the value drops below 1% and thus protection is guaranteed, but this also involves high costs.

Since the private end consumer now expects a nickel-free product, it is recommended that costume jewelry be plated with a strong gold plating. An XRF is then performed and the nickel content is measured in addition to the precious metals and heavy metals.

testxchange: In the trade with gemstones, their origin is often of interest, for example to rule out the possibility that they come from countries with inhumane mining conditions. What possibilities can a testing laboratory offer to create more clarity here?

Andreas Stratmann: Different geological formation histories (metamorphic or volcanic origin) caused typical inclusion types in gemstones for certain regions, which we can examine in microscopy. Thus, there are three-phase, gas, and liquid inclusions, as well as healing cracks, plumes and feathers, and mineral inclusions. In addition to inclusions in gemstones, certain optical phenomena may also be characteristic. In addition to gemstone microscopy, trace elements also provide clues to the origin. For example, an emerald from Colombia has a higher vanadium-chromium ratio with respect to the color-giving elements.

In summary: Via gemstone microscopy and the determination of the chemical composition by means of X-ray fluorescence analysis, the origin can often be determined. However, the gemstone to be examined must have a certain minimum size to be able to find inclusions at all. In the case of set stones, the examination is severely limited.

In rare cases, gemstone dealers have certificates of origin from the corresponding mines. For example, mine operators in Tanzania have the local mining authority issue a certificate of origin together with an export permit. This certificate makes further examinations unnecessary and, together with the purchase invoice of the import VAT declaration (customs documents), serves as complete proof of origin.

testxchange: You also offer seminars on gemstone identification and a gemstone database for X-ray fluorescence analyses. What qualifications should interested parties have in order to take advantage of such offers and to be able to test gemstones themselves?

Andreas Stratmann: A certain level of technical understanding, spatial awareness and a general interest in science would be helpful. Of course, it is always good to have previous knowledge, but this is not absolutely necessary.

Versatile test instrument for various analytical methods: the UV-A/VIS/N-IR gemstone spectrometer

testxchange: In recent years, you have developed your own gemological testing equipment, including a UV-A/VIS/N-IR gemstone spectrometer. Are gemological tests generally carried out predominantly according to specific test standards, or are the test procedures here more individual, depending on specific requirements?

Andreas Stratmann: In addition to the well-known standards and requirements for a testing laboratory, as an expert you are bound by the rules of the SVO (German Expert Regulation) and as a control body you are subject to the chambers and the Expert Council.

In the field of gemological examinations we follow the rules of CIBJO (International Association of Jewelry, Silverware, Diamonds, Pearls and Stones, also called World Jewelry Confederation) and the rules of IDC (International Diamond Council).

In America, the GIA (Gemological Institute of America) declares its own rules on nomenclatures and in China there is something like a supreme appraiser to whom all others are subordinate. The rest of the world works according to the rules of CIBJO and IDC.

A particular challenge in our field is the fact that only non-destructive examination methods may be used. The approximately 70 types of gemstones are assigned to individual gemstone groups and divided into 7 crystal systems according to their optical properties, which enables systematic determination. Nothing is left to chance here and nothing is "individual".

Systematic gemstone determination with gemological equipment and modern examination methods is the term to refer to the following test procedures, with all examinations to be performed under 5500 Kelvin daylight. For tests with long-wave UV-A light 365 nm and for tests with short-wave UV-C light 256 nm are to be used:

The research of optical properties by means of polariscope and dichroscope. Here, it is determined whether a stone is isotropic, anisotropic uniaxial or biaxial, or microcrystalline. A thermal resistance measurement is performed and the reflectivity number is determined.

If possible, the gravity is checked and compared with the weight and dimensions. The critical angle of the total internal reflection is the value of the light refraction, which is measured with a gemstone refractometer. The light refraction is determined again with a digital reflectometer.

Under long and short wavelength ultraviolet light, luminescent phenomena are studied. By means of a magnetic resonance balance, the response in a magnetic field is investigated. Possible treatments by radioactive irradiation are revealed with a dosimeter by measuring the radioactivity in microsieverts/hour.

For a possibly desired valuation, the weight in carats (metric carat, abbreviated ct) and the dimensions must also be determined, and the cut, proportions and color must be evaluated. For this purpose, there is a proportion scope, a standardized color evaluation system and color comparison stones for diamonds. After the gemological data has been determined, microscopy is performed. For this purpose, a gemstone microscope with a large dark field condenser, stone holding device, polarizing filters, transmitted light, reflected light and oblique light is used.

My UV-A/VIS/N-IR Gemstone Spectrometer© is the further development of the traditional spectroscope. With the spectroscope only the visible color spectrum is examined. One looks at a small rectangular colored spectrum, which is then to be compared with the technical literature. Investigations with the spectroscope are not very diagnostic and sobering, which is due to the fact that absorption bands, instead of lines, are observed for solid substances. Since there was no spectrometer available that also allows investigations on set and small stones, I developed the UV-A/VIS/N-IR Gemstone Spectrometer© and created an extensive reference database.

In addition to the visible VIS spectrum, UV absorption, the fluorescence spectrum triggered by UV laser radiation, and absorption in the near infrared range are measured. In addition, IR reflectance can be used to detect specific treatments. The instrument achieves a measurement accuracy of 0.1 to 0.5 nm, depending on calibration. The spectra determined with the gemstone spectrometer are collectively very informative. Another special feature is that the spectra of the database are independent of the gemstone spectrometer, which is not self-evident. The VIS spectra correspond to the spectra measured with conventional spectroscopes and can also be matched with existing literature if required.

A Raman spectrometer is used to test the vibrational behavior at the molecular level. The Raman spectrum is also very diagnostic for minerals and can also reveal coated areas and types of treatment with oils and polymers.

X-ray fluorescence (XRF) analysis is used to determine the chemical composition at the elemental level. Using a modified dental X-ray unit, the internal structure of pearls can be visualized, and lead-doped crack fillings in gemstones can also be made visible.

A UV-C reflectance spectrometer equipped with a deuterium source allows investigations also on set very small diamonds to detect HPHT treatments and syntheses. A diamond screening device is also available for the detection of type IIa diamonds.

XRF is available for testing gold and other precious metal alloys. In the case of gold coins and gold bars, magnetic resonance can be used to check whether a gold bar has a core of base metal, for example tungsten, which has the same density as gold. In addition, we can measure the different sound velocities of metals using an ultrasonic meter. XRF can also be used to make statements about the age of historical jewelry, since in the past it was not possible to process the raw gold found into almost chemically pure gold with a fineness of 999.9/000 Au.

For the interested readers I would like to finally refer to my books. The Gemstone Encyclopedia Part 1: Systematic gemstone identification with gemological equipment and modern examination methods, as well as the Gemstone Encyclopedia Part 2a Agate-Corundum: The gemstone species with gemological data, as well as pictures of the spectra and microscopy.

The technical books were created from my script for teaching apprentices in the goldsmith trade and describe the equipment, its operation and application in a generally understandable way.

testxchange: Mr. Stratmann, thank you for the interview!