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lGemlab. UK  is a Gem Tutorial Centre allied to the Gemmological Association of Great Britain.
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SPECTRA DATABASE

INTRODUCTION

This database is mainly for use by students studying gemmology in the courses offered by the Gemmological Association of Great Britain and the National Association of Goldsmiths in the United Kingdom. In these courses a sound knowledge of gemstone spectroscopy is essential, together with the ability to recognise the absorption and fluorescence spectra of many of the gem materials included in their syllabus. It is hoped that Gemmologists and those working with and trading in gemstones from other countries, may also find it useful as a general reference guide when comparing spectra of gemstones, collectors minerals and other material for identification. The database will eventually include common, unusual and rare gemstones, synthetic and artificial gemstones. Other sections will include rare collectors minerals and artificial materials used in modern technology. There will also be sections dealing with filters, liquids and the emission spectra of light sources used in the examination and testing of gem material.

As the work is extensive it will be released in sections to which extra items can be inserted periodically as they are recorded and photographed.

THE SPECTRA AND THE SPECTROSCOPE

The majority of the spectra have been photographed through a small OPL diffraction spectroscope attached to a digital camera and coupled to the body tube of a monocular microscope. The use of this set-up is twofold. In photography it ensures maximum control to acquire an image and in teaching it provides a stable image which students can view without having to handle the specimen or the spectroscope. This is hands free spectroscopy.

The use of a monocular microscope in conjunction with a spectroscope is not a new idea, but often the spectroscope was simply balanced on top of the eye tube and the light source was directed vertically from below via a substage condenser into the gemstone. This works adequately with a flat tablet with two parallel faces or perhaps even a cabochon, but not with a faceted stone. In this method the majority of the light is scattered or lost by total internal reflection, especially with stones of high refractive index, and fails to enter the spectroscope. To overcome this I have developed a method by which the light is directed horizontally above the microscope stage and enters the faceted stone via the pavilion. This is set at a suitable angle to enable internal reflection from the table facet and maximum light is then directed up into the body tube of the microscope and on to the slit of the spectroscope.

In the case of very small stones I use an objective lens with a working distance of 50mm as a light gathering accessory in which case true optical alignment is essential.

THE GEMSTONE SECTION

This has to be easily accessed and used by students. After much deliberation and discussion with colleagues I have simply arranged it by colour, which is the primary consideration in any initial investigation with a gemstone.

There is unavoidable repetition of similar spectra which occur in the different hues, or even in the different colours, of certain gemstones. However slight variations may occur which can be important features in identification. Although most specimens of a particular species will show a similar pattern in their spectra, the student must realise a gemstone spectrum can be unique in that not all of the absorption or fluorescent lines may be present to the same degree in all specimens This is due to several factors such as colour saturation, size of stone, transparency and colour temperature and intensity of the light source.

Unlike many drawings, diagrams and coloured illustrations, these photographs reveal what a student may expect to see through a hand held direct vision diffraction spectroscope. In some cases the absorption lines may be strong and bold, in others they may be weak and vague just as we often find them. There are some spectra in which the camera has failed to resolve some of the finer lines and others where it has revealed certain lines which are seldom detected be eye, such as those in the violet region.  The limitations of digital colour photography can affect different areas of the spectrum such as a lower intensity in the yellow and an increase in the green. However, the overall impression is generally as the student should see the images. In some cases it has been necessary to over expose in the deep blue – violet in order to resolve some lines in that area, hence the imbalance of colour in some photographs.

                                                                                   CALIBRATION

In most cases calibration was checked using a Beck prism spectroscope No.2522 with built in wavelength scale and tolerance allowed due to parallax limitation. In the case of some unusual, rarely encountered materials, spectra of known wavelength were introduced into the optical path during photographing. This was done using thin sections of materials doped with rare earth elements,(neodymium, erbium and thulium); certain liquids (Potassium permanganate and olive oil); and certain light sources (sodium, mercury vapour and tri-phosphor coated tubes.) The method used was to superimpose one of these spectra on to the spectrum of a specimen being photographed. From there calculations were made between the known and unknown positions of the lines in the spectra using a nanometer- pixel related formula by computer.

                                      

COLOURLESS AND NEAR COLOURLESS GEMSTONES CS1

RED GEMSTONES - RD1(Dark red);   RD2 (Pinkish red);  RD3(Orangy red);

RD4(Slightly purplish red);   RD5(Purplish red):

These are the first sections to be uploaded and additional gems will be inserted to them as they are processed.

Click here to view Excel files  CS1 ; RD1 ; RD2; RD3; RD4; RD5:
See my  DECARATION OF COPYRIGHT

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