A gemmologist commonly uses hand-held or desk model spectroscopes to observe the absorption features of a gemstone, in the visible region 400 - 700 nm. However, human eyes are not capable to observe very minute features, which are missed to be seen and hence, may affect the conclusions and overall results. Therefore, spectrometers or spectrophotometers may be used, which are capable to analyse the sample in detail and registers the absorption features in the form of a graph. These spectrometers not only analyses the sample within 400 - 700 nm range, but much beyond this, from 175 nm in the ultra violet region up to 3300 nm in near-infra red region. This however depends on the type and grade of instrument and accordingly, the spectrometers can be termed as UV-Vis or UV-Vis-NIR spectrometers.
In UV-Vis-NIR spectrometry, the scale or position of absorptions is expressed in wavelength nanometers (nm) as against the IR or Raman spectrometers, using wavelength inverse (cm-1). The UV-Vis-NIR spectrometry is based on selective filtering of wavelengths through the sample. The absorbed energies or wavelengths are displayed in the form of a graph and expressed in comparison with the incident light i.e. original light emitted from the source.
In general, double beam spectrometers are used, where a beam splitter divides the incoming light beam into two beams, one for the sample and another for a reference (known as reference beam). A typical spectrophotometer contains a light source, which may be deuterium and tungsten or a xenon lamp; monochromator, which is basically a wavelength selector and selects the required wavelength from many wavelengths emitted from the source, as per the application; and detectors, which are usually photomultiplier tubes used to measure the dispersed components of the beam after it passes through the sample.
Schematic principle of a typical double beam UV-Vis spectrophotometer
Absorption features in the UV-Vis region are in general caused due to electron defects or presence of chromophoric elements (colouring elements), while those present in near infra red (NIR) region are due to the vibrations of the molecular groups present. Therefore, the spectrophotometry provides detailed information about the cause of colour in a sample, thereby assisting in determination of a gem material, separating natural from synthetic gem materials, identifying treatments, etc. It not only determines the trace element, but also its type, for example, whether Fe is present as Fe2+ or Fe3+.
The Gem Testing Laboratory, Jaipur uses an Agilent Cary 5000 double beam UV-Vis-NIR spectrophotometer, which is capable to scan a sample from 175 nm to 3300 nm within few minutes.
Such type of instrument is highly sensitive and detects even smaller and weaker features. When taking spectra of isotropic minerals, such as spinel, fluorite, diamond, etc, the beam can be transmitted through the sample in any direction, however, in case of anisotropic minerals, the direction of transmission of beam plays an important role, because of the pleochroism or asymmetry of crystals. For uniaxial crystals, two principle directions are followed i.e. optic axis (along vertical 'c' axis) and perpendicular to the optic axis (horizontal crystal axes) or in other words directions of 'ordinary' and 'extraordinary' rays are measured. This is done by positioning the optic axis of the stone perpendicular to the path of beam and then using a polarizer to collect the spectra, in parallel and perpendicular directions. In case of biaxial crystals, the samples have to be studied thoroughly before collecting the spectra. For biaxial crystals, it is relatively easier in rough samples to orient properly for directional analysis, as compared to cut and polished stones.
For opaque samples, diffused reflectance accessory is being used.
In case of diamonds, spectra are also collected at liquid nitrogen temperature (LNT) of approximately -160oC. This provides much sharper and clearer spectra as compared to that taken at room temperature. This becomes more important when origin of colour of a diamond is to be determined.
At Gem Testing Laboratory, UV-Vis-NIR spectrometer is used to:
Identify a gem species / variety, especially when separation of close look-alike gem varieties or species becomes very difficult. This is done by studying the presence of trace elements and their characteristic features. The spectrometer has proved useful in separation of emerald from green beryl on the basis of absence or presence of chromium and/or vanadium; it also helps in detection and classification of types of garnets especially in case of intermediate varieties, detecting paraíba-type tourmaline and much more.
Absorption spectra of green beryl (red trace) and emerald (blue trace).The two green gem varieties of beryl are differentiated on the basis of chromium and vanadium related absorption bands and peaks.
UV-Vis-NIR spectra help to identify and differentiate copper-bearing tourmaline (red trace) such as those from Paraíba, Nigeria or Mozambique from non-copper bearing tourmalines (blue trace). The band at 918 nm is due to Cu2+, which causes the bright neon colours in these tourmalines.
Separate natural and synthetic gem material, as in some cases the exact cause of colour in natural and synthetic gems may vary slightly, because these synthetic materials might not be replicated exactly with minutest of details as their natural counterparts. And such minute differences can be measured by the spectrometer. This is commonly used to differentiate natural from synthetic sapphires or spinels, ceramic imitation from natural turquoise or coral, etc. Due to the use of the wavelengths in the near infra-red region, separation of natural and synthetic quartz can also be done.
Natural sapphires, especially from the basaltic environment display strong iron-band at around 450 nm with associated weaker bands at 460 and 470 nm (red trace), while those from metamorphic environment show a single band at 450nm; these features are not present in a synthetic counterpart (blue trace).
Separation of natural (red trace) and synthetic (blue trace) spinels, becomes quite conclusive due to the presence of iron band at around 460 nm in natural spinels, even if the stones are clean.
Detect treatments, in case of artificial colouration of diamonds or dyeing of corundum, beryl, quartz, turquoise, corals, opals, etc; very useful for determining thermal colour fusion (diffusion) or coating in topaz or tanzanite.
Natural turquoise (red trace) can easily by differentiated from dyed turquoise (blue trace) by observing the Fe3+ peak at around 430 nm in UV-Vis spectra , which is absent in dyed counterpart of the similar colour appearance.
Coating, diffusion or thermal colour fusion on topaz to produce various colours has gained a lot of popularity in the recent past. UV-Vis spectra help to identify and differentiate these methods of artificial colouration from natural colours or other stable forms of treatments such as heating and/ or irradiation. Here shown is an irradiated blue topaz (red trace) and a diffusion treated or thermal colour fusion treated blue topaz (blue trace), which is coloured by cobalt, as revealed by three peaks at 551, 585 and 623 nm.
UV-Vis spectra also assist in origin determination, by studying the type of trace element present, such as separating sapphires from basaltic (e.g. Thailand) and metamorphic (e.g. Burma) environments; similarly, separating emeralds of Zambian origin from Columbian, although, the stones belonging to similar geological environment tend to overlap in properties, and hence, currently the test is not conducted at the Gem Testing Laboratory.