SPECTROSCOPIC STUDIES ON NATURAL, SYNTHETIC AND SIMULATED RUBIES Ms Low Yee Ching Supervisor: Assoc Prof Augustine Tan T.L. Natural Sciences Academic Group National Institute of Education 1 Nanyang Walk, Singapore 637616 tltan@nie.edu.sg Keywords: Rubies, gemology, UV-fluorescence, FTIR, EDXRF Abstract Ruby is a variety of the mineral species corundum and has been a popular gemstone for centuries. Thought to represent the energy of the sun in the past, ruby is well known and expensive. With its characteristic colour and high hardness, its value is unrivalled by any other gemstones except a few rare colou diamonds. Historically, gem possession has been reserved for wealthy, royalty or high religious leaders. Being human nature to want what others possess, imitation rubies have been in existence for some 4,500 years. Advances in crystal growth technology have made the creation of synthetic rubies a reality. Synthetic rubies can now be grown through hydrothermal, flux and melt method. As a result, there exist numerous synthetic and simulated rubies in the gem market which are not easily identified through use of basic gemological methods such as optical microscopic observations. In this research exercise, spectroscopic techniques such as energy-dispersive x-ray fluorescence (EDXRF) and Fourier transform infra (FTIR) were employed to complement other non-destructive tests to distinguish between natural, synthetic and simulated rubies. 1. Introduction Red corundum (Al 2 O 3 ), more commonly known as Ruby is a well known and much sought after gemstone in the world. The colour in the stone was due to the presence of trace elements such as chromium (Cr), iron (Fe) or titanium (Ti). In medieval Europe, rubies were worn to guarantee health, wealth, wisdom, and success in love. Large fine quality rubies are extremely rare and valuable. Some large Spinel were even mistaken to be fine quality rubies and set onto British crowns. Due to the high demand, materials such as glass and Cubic Zirconia were used as imitations for rubies. Other stones such as Spinel, Garnet, Tourmaline, Agate and even colou quartz were marketed as Ruby. Advancement in technology has brought about the creation of synthetic rubies. Various techniques, such as hydrothermal, flux and melt, produced such good synthetic rubies that 1
requi Fourier transform infra (FTIR) spectroscopy and energy-dispersive X-ray fluorescence (EDXRF) spectroscopy to complement traditional gemological tests. A simulated ruby is usually and clear. Tests for physical properties such as refractive index, specific gravity, hardness and birefringence may be effective in identifying simulated rubies but not the synthetics. Essentially having the same composition (Al 2 O 3 ), synthetic and natural rubies have similar physical properties. Hence, it requires a very skilled person or highly sophisticated equipment for positive identification between natural and synthetic rubies. Marketed as a gem, rubies are mostly sold with close metal mountings. Non-destructive techniques are requi for these rubies. The metal mountings made tests for physical properties such as refractive index and specific properties impossible and microscopic observations for tell tale inclusions may not be accurate. For such cases, non-destructive spectroscopic methods such as FTIR and EDXRF are very useful. In this research exercise, FTIR, EDXRF, and other traditional gemological tests for refractive index, UV fluorescence, specific gravity and the dichroscope, were used to identify natural, synthetic and simulated rubies. 2. Materials and Experiments Five samples of natural rubies (samples 1 to 5), three samples of synthetic rubies (samples 6 to 8) and three samples of simulated rubies (samples 9 to 11) were used in this research exercise, as shown in the figure on the title page. The samples were labelled 1 to 11 (from left to right, top to bottom) in the figure. Samples 1 to 4 originated from Vietnam, sample 5 from Thailand and samples 6 to 11 from a commercial source. The photograph shows five natural rubies (top row, from left: samples 1 to 5), three synthetic rubies (middle row, from left: samples 6 to 8), and three simulated rubies (bottom row, from left: samples 9 to 11). 2
First, the samples were observed under the optical microscope built for gem identification to study colours and inclusions of the rubies. The refractometer was used to determine the refractive index of the samples. A small drop of refractive index liquid was placed on the hemicylinder to eliminate air between the stone and glass so that optical contact was established between the two surfaces. With a white light in front of the refractometer, the refractive index of each sample was measu with an accuracy of ±0.005. All the samples were viewed through a dichroscope. A sample was placed near the end of the dichroscope while pointing at a white light source. The colours displayed in the two windows in the dichroscope were observed and noted. The colours would show the presence or absence of dichroism in the rubies or other stones. The fluorescence of the samples under short-wave (254 nm) and long-wave (365 nm) ultraviolet (UV) radiation were also performed. The samples were thoroughly cleaned with ethanol prior to the FTIR experiments to remove any trace of organic contaminant on the surfaces due to handling. 16 accumulations of the IR spectra of each sample in the wavenumber range of 400 4000 cm -1 were recorded. All samples were directly used without destructive processing. The clear samples allowed infra light to pass through them and be absorbed. The infra transmission spectra were then recorded to give absorption peaks for identification of materials. The accuracy of the absorption peak is accurate to ±1 cm -1. All the samples were also tested using the energy-dispersive X-ray fluorescence (EDXRF) spectroscopic technique. In this method, the elements in each sample were detected. Surface analysis by EDXRF is accomplished by bombarding a gold-coated sample with high energy electrons and detecting and analysing the energy of the emitted X-rays. The samples were first coated with a layer of gold to prevent charging up. The electron beam has penetrating power in a solid extending 1 to 5 µm, depending on the energy and the nature of the substance. 3. Results and Discussions The microscopic observations and description of the stones, results on refractive index, dichroism, UV fluorescence, FTIR as well as EDXRF for all 11 samples are tabulated in Table 1. 3.1 Microscopic Observations The presence of crystal inclusions of irregular shapes and long fissures in samples 1 to 5 indicate that they are probably untreated and natural. Opaque inclusions and negligible fissures, which are typical of synthetic rubies, can be seen in samples 6 to 8. For the simulated rubies, no inclusions can be found in samples 9 and 10, while isolated crystal inclusions exist in sample 11. 3.2 Refractive Index Ruby belongs to a class of mineral that is anisotropic. It demonstrates the phenomenon of double refraction, which is also known as birefringence. Samples 1 to 8 have refractive indices ranging between 1.77 and 1.785, which agree with published data for natural and synthetic rubies. Samples 6 to 8 demonstrated the phenomenon of double refraction. Though 3
samples 1 to 5 are double refracting, only one refractive index reading could be obtained as the inclusions in them rende the two refractive indices too close to be measu. A reading of 1.54 obtained for sample 11 suggested that it is an agate, while samples 9 and 10 have refractive indices that are too high to be measurable by the refractometer. 3.3 Dichroism The dichroscope is one of the quickest and easiest ways to identify simulated gems. Samples 1 to 8 displayed the characteristic orange- and purple- for rubies in the two windows, while samples 9 to 11 showed no change of colours in the two windows. However, one drawback of this technique is its inability to differentiate between natural and synthetic stones. Hence, other tests are requi to complement the dichroscope. 3.4 Ultraviolet (UV) Fluorescence All natural and synthetic rubies exhibit various shades of fluorescence under long and short waves of UV light, which are true for samples 1 to 8. This test is effective in isolating samples 9 to 11 as being non-rubies, for all three samples were completely inert to both long and short waves of UV light. 3.5 Fourier Transform Infra (FTIR) Spectroscopy The detection of absorption peaks in this technique is useful in differentiating natural rubies from synthetic or simulated ones. All four natural Vietnamese rubies (samples 1 to 4) were found to have one infra absorption peak at 2109 cm -1 and the other around the 1971-1986 cm -1 region. A broad absorption region (2600-3800 cm -1 ) was also detected in all four spectra. The natural Thai ruby (sample 5) had a different infra spectrum, with weak infra peaks at 2435, 3232 and 3304 cm -1. All three synthetic rubies (samples 6 to 8) had several weak absorption peaks in the 2800-3400 cm -1. Since all the spectra of the synthetic rubies are different from those of the natural rubies, differentiation of the two types can be done accurately. For the simulated rubies (samples 9 and 10), a infra absorption peak at 2087 cm -1 was detected. The last simulated ruby (sample 11) was almost opaque to infra radiation and so, the absorption spectrum could not be obtained. 3.6 Energy-Dispersive X-Ray Fluorescence (EDXRF) The EDXRF spectra of natural and synthetic rubies exhibit X-ray peaks of aluminium and oxygen, indicating basic Al 2 O 3 composition, with negligible trace elements. EDXRF spectra for simulated rubies (samples 9 and 10) show peaks for zirconium (Zr) and oxygen (O), indicating that the samples are basically zirconium oxide (ZrO 2 ) or commonly known as cubic zirconia. For simulated sample 11, silicon and oxygen were detected, showing that the chemical composition is SiO 2 which is quartz material. It is most probably carnelian agate. 4 Conclusion The FTIR and EDXRF spectroscopic techniques were found to be useful scientific tools in identifying the natural and synthetic rubies from the simulated ones. 4
Table 1. Gemological and spectroscopic data on natural (samples 1 to 5), synthetic (samples 6 to 8) and simulated (samples 9 to 11) ruby. Sample Description Refractive Index 1 Natural 2 Natural Specific Gravity 1.78 4.11 Strong 1.78 4.11 Strong Ultraviolet Fluorescence Long Short wave wave Infra absorption peaks (cm -1 ) Red 1461 1974 2108 3094 3304 Red 1468 1747 1985 2108 3072 EDXRF Elemental peaks 3 Natural 4 Natural 5 1.65 carat genuine round clear deep untreated Thai ruby. 6 Oval clear deep stone 7 Oval clear deep stone 8 Oval clear deep stone 9 Oval clear deep orange stone 10 Oval clear orange stone 11 Oval flat translucent deep orange cabochon 1.785 4.11 Strong 1.785 4.11 Strong 1.77 4.83 Strong milky 1.76 /1.77 3.64 Strong 1.76 /1.77 4.71 Strong 1.76 /1.77 3.60 Strong Not available Not available Red 1432 1739 1971 2108 3086 3181 3275 Red 1219 1382 1743 1981 2108 2340 3065 1067 1176 1291 1407 1580 2934 3231 3304 865 959 1038 1299 1407 1580 2362 2920 3231 963 1093 1252 1324 1461 1205 1342 1432 1494 1588 1678 2920 3231 6.44 Inert 934 1082 1353 1454 2086 2920 5.69 Inert 890 963 1096 1342 1432 2086 2905 Zr, O Zr, O 1.54 2.65 Inert No results Si, O 5