Analyzing Gemstones in Jewelry with FT-IR

Synthetic gem-crystals are available since the late 1800s and their production processes were more and more refined. Later, starting in the 1950s, diamond was available via different types of syntheses and it is now possible to produce them in gem-quality with individual masses of one carat or more. Many different methods were developed in parallel for the quality improvement of natural gemstones and diamonds like fracture filling, color improvements via physical and chemical methods and so forth. Hence, understanding the stone’s genesis and history has become important and has significant economic implications.

To identify and classify diamonds and gemstones, Fourier-transform-infrared spectroscopy (FT-IR) is one of the most important analytical techniques and the method of choice. However, analyzing mounted stones which are very small or in close proximity to other stones is often a difficult task because the commonly used reflection techniques lack the spatial resolution required for such difficult specimens. The LUMOS FT-IR microscope can analyze mounted stones of almost any size. Thanks to its high working distance, the LUMOS can analyze such stones that cannot be accessed by macroscopic IR-measurement techniques like, for instance, sunk-in stones.

Figure 1. LUMOS FT-IR microscope.

Instrumentation

The unparalleled ease of use, convenient automation, and excellent visual and infrared spectroscopic performance are the obvious benefits of the LUMOS FTIR microscope. It is possible to inspect the mounted stones using a special sample holder vice that allows all types of jewelry to be fixed. Using the vice, samples can be tilted up to a certain angle to precisely position different measurement points without removing the sample.

To make the operation of the automated hardware even easier, OPUS provides assistant-guided measuring procedures. The dedicated OPUS video-wizard – a user interface that always provides the appropriate functions for the current measurement step – helps the user through the whole measurement procedure. Even though the LUMOS can be operated by novice users for routine applications, the outstanding sensitivity of the instrument makes it also suitable for high demanding applications.

Example: Diamond Ring

The sample holder vice with a diamond ring is shown in Figure 2. The whole ring is illustrated in the picture inset. The single diamond stones are very small and mounted very closely together which poses a challenge to analyze the stones individually with macroscopic IR-measurements. Figure 3 shows an example measurement of one of the stones along with the visual image. The IR-spectrum allows direct conclusions to be drawn about the crystal lattice of the diamond and facilitates detection of impurities like hydrogen, boron, or nitrogen.

Also, information about the configuration of the nitrogen impurities like the presence of isolated nitrogen atoms or clusters of two or four nitrogen atoms can be obtained. The x-axis of the IR spectrum shows the reciprocal value of the wavelength in cm^-1 (i.e. how many waves present over one cm) which is a unit widely used in IR spectroscopy, and the y-axis the absorption. Nitrogen impurities normally exhibit absorptions between approximately 500 and 1500 cm^-1, whereas the typical peaks of boron impurities can be observed around 2800 and 2460 cm^-1.

The strong and broad bands between approximately 1600 and 2700 cm^-1 are the so-called two-phonon bands and characteristic for all diamonds. They are a brilliant marker that can be used to distinguish between diamonds and imitates. In addition, based on the type and configuration of foreign atoms, a specific type can be assigned to every diamond. This type of information helps to distinguish between natural and synthetic stones. Figure 3 shows a typical spectrum of diamonds of type IaA which demonstrates the natural origin of the stone. For diamonds in jewelry, the spectrum typically shows additional signatures from organic contaminations like, for example, from skin fat. The double peak below 3000 cm^-1 is due to the C-H stretching vibrations that are characteristic for organic substances.

Figure 2. Diamond ring fixed in the sample holder vice on the LUMOS stage.

Figure 3. Diamond spectrum of one of the stones from the ring.

Example: Gold Ring with Gemstones

Figure 4 shows an example of the analysis of different stones mounted in a gold ring. Among others, the ring contains three very small sunk-in colorless stones which are very difficult to access. A visual image combined with the IR spectrum of one of the clear stones is shown in Figure 5. From the spectrum, it is evident that the stone is a diamond from type IaAB which confirms its natural origin.

Figure 4. Gold ring with diamonds and green emerald.

The spectrum also presents additional bands from protein and fat that are superimposed on the regular diamond spectrum.

Furthermore, spectra were measured from a representative green, blue and red gemstone mounted in the ring (Figure 6). As shown in Figure 6, the two upper spectra are typical reflection spectra of corundum and stem from the blue and red gemstone which can, therefore, be identified as sapphire and ruby. The bottom spectrum in Figure 6 is from the green stone, showing bands that are characteristic for beryl. Therefore, the stone is identified by the spectrum and its green color as an emerald.

Figure 5. Image and spectrum of one of the diamonds from the ring pictured above.

Figure 6. Spectra of gemstones in the gold ring. From top: Blue, red and green stone.

Summary

The LUMOS FTIR microscope can analyze, identify, and classify even very small mounted diamonds and gemstones and is also capable of handling sunk-in stones. It can even measure small stones that are mounted very closely together with other stones with a very high selectivity. From the spectra data collected, conclusions can be drawn whether the stone is treated or synthetic and thereby evaluating its true value.

This information has been sourced, reviewed and adapted from materials provided by Bruker Optics.

For more information on this source, please visit Bruker Optics.