Raman spectroscopy, as the name suggests is based on the Raman Effect discovered by the Indian physicist, Sir Chandrashekhar Venkata Raman (C. V. Raman) in 1928. Raman Effect takes place when monochromatic light (usually a visible laser) strikes a molecule and interacts with the bonds of that molecule. While most of the incident laser light scatters and produce no useful information (known as Rayleigh scattering), some of the light is re-emitted, which is shifted to longer wavelength with a loss of energy, known as 'Stokes radiation'. This shifted or stokes radiation produces a characteristic spectra or pattern of peaks, which can be considered as a 'fingerprint' for that substance.
Raman spectroscopy complements the infra red spectroscopy very well and provides useful information where FTIR fails to answer many questions. It is fast and non-destructive method of analysis without any sample preparation. In addition to the H2O or CO2 content of the sample, it also identifies the chemical groups irrespective of the thickness of the sample and without taking a powder on the sandpaper.
A typical Raman spectrometer unit contains:
Schematic principle of a typical Raman Spectrometer
Because of the use of powerful microscope, Raman spectroscopy can be performed on very small surfaces of micron sizes and hence it provides a versatile application in various gemmological analyses. It not only presents the information about the host mineral, but also inclusions present in it, or the various layers within, as in case of composites, or colour zoning, and much more. It proves to be very useful in case of large specimens or gems set in a jewellery, or when the size of stones are too small to conduct standard gemmological tests, or if the sample is in rough form. The spectra thus obtained are carefully compared with the database to draw the results. A detailed and comprehensive database is always required to carry out a successful Raman analysis.
The Gem Testing Laboratory, Jaipur, uses an Airix Corporation / TechnoS Instruments (formerly Seki Technotron Corp. / Cornes Technologies Ltd) STR 300 Raman system, equipped with two wavelength lasers of 532 and 785 nm and a Peltier cooled CCD detector. The laser light is focussed and collected from the sample through an Olympus microscope, equipped with 5x, 20x, 50x and 100x objective lenses, assisting analyses of very small areas. The transmission of laser and scattered Raman signals is done through highly sensitive optic-fibre cables, in place of traditional mirror arrangements. Such configuration provides features for even very weak Raman Effect.
At the Gem Testing Laboratory, Laser Raman Spectrometer is mainly used:
To identify an unknown gem material, especially when other properties closely overlap, such as tourmaline and amblygonite, or enstatite and diopside, or in case of gems exhibiting isomorphism and polymorphism; black diamond and black moissanite when set in a piece of jewellery; components of a rock mixture, such as occurrence of ruby with fuchsite and kyanite, can be identified conclusively by this method, as all these have a specific 'fingerprint' pattern.
Black diamond (red trace), black moissanite (blue trace) and black cubic zirconia (black trace) can easily be separated by Raman spectra, even if they are set in a jewellery.
Raman spectra of tourmaline (red trace) and amblygonite (blue trace) are quite different, assisting in their separation, which otherwise are quite challenging to differentiate.
Components of rock mixtures such as this ruby-fuchsite-kyanite rock can easily be identified conclusively.
To analyse internal features, using the high power microscope, by which exact nature of the mineral or fluid inclusions can be identified, thus assisting in determination of origin and in many cases separating natural and synthetic rubies and sapphires or emeralds. In many cases, study of inclusions helps to detect treatments, especially 'low temperature' heating in rubies and sapphires. This is also helpful in identification of small sized stones and diamonds set in jewellery or luxury watches.
Detection of ' low-temperature heating' of rubies and sapphires is quite challenging by simple microscopic analysis as the inclusions present are not affected much. Zircon crystals, which are one of the most common inclusions present in rubies and sapphires, provide very useful information regarding heating. Zircon crystal when present in nature are usually in metamict state (almost amorphous), hence do not display sharp peaks. However, on heating, zircon restores its crystalline state, and as a result, the intensity of absorption peaks also increases (blue and red traces), assisting in detection of even low-temperature heating of below 800-1000oC.
To determine treatments in gemstones, especially, polymer-impregnated turquoise or jadeite, fracture filling of emeralds, dyeing of corals, glass-filling in rubies, coatings in various gemstones, low temperature heating in rubies and sapphires (by analysing the inclusions), high pressure high temperature (HPHT) in diamonds (using Photoluminescence method).
Natural red colouration in corals is due to the presence of carotenoids and parrodines which can be detected in Raman spectra (red trace). These are missing in dyed counterparts (blue trace); hence, separation becomes quite conclusive with Raman spectra.
In addition to FTIR spectroscopy, Raman analysis also helps to detect the presence of Resin (blue trace) in fracture of an emerald (red trace). Note the distinct peak at 1609cm-1, associated with resin.
Polymer impregnation is a common practice to provide better polish and hence improve the lustre of a turquoise. This can conclusively be identified by Raman spectroscopy. The blue trace is of a polymer-impregnated turquoise, while red trace is of a natural.
To check photoluminescence spectra, which is helpful in determining the cause of colour, the presence of colouring agent and hence, identifying the stone. Photoluminescence or PL spectra can be collected at room temperature or at the liquid nitrogen temperature (LNT), depending on the type of sample and information required. Spectra collected at LNT are sharper, clearer and provide much more information than at the room temperature. The spectra thus obtained, can be used to determine the presence of treatments, such as HPHT in diamonds or heating in spinel, or to differentiate natural and synthetic gemstones, such as spinel, diamond, etc.
Room temperature photoluminescence (PL) spectra of ruby (red trace) and red spinel (blue trace) assisting in easy separation even when stones are set in jewellery