Gemlab.UK Gemmological Spectroscopy SPECTRA DATABASE
VIOLET AND PURPLE STONES
Periodically other gemstones will be added to these files
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 Jewellers
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 GEMSTONE SECTIONS
GEMSTONE SPECTRA COMPARISON follow the links in column on the left to view the EXCEL FILES
These have 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.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.
GEMSTONES follow the links in the column on the right to view their individual
THE VISIBLE SPECTRA IMAGES
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.
THE SPECTROPHOTOMETER GRAPHS
Although my Spectra Database is mainly devoted to visible diffraction spectra it is becoming increasingly important that the student of gemstone spectroscopy should be aware of the relationship between the direct vision spectroscope and the spectrophotometer. Sadly it is often stated that the spectrophotometer now replaces the spectroscope which, from the average gemmologist's point of view is, untrue. However for those who are used to working with a spectrophotometer I will introduce my results periodically as my database progresses. These I will refer to as Spectrographs and the instrument as a Spectrometer.
I produce my initial spectral scans on a GL Gem Spectrometer www.gemlab.ws this gives a plot of the spectral curve in which the vertical Y axis is calibrated as the intensity of either transmittance or absorbance and the horizontal X axis as wavelength in nanometres. This scan is saved as a graph and then further processed using “Spectragryph” spectrometer software. www.spetroscopy.ninja Further editing enables me to include my digital images of the actual gemstone and its visible spectrum as seen through a diffraction type direct vision spectroscope. For easy reference the X axis (wavelength scale in nm.) on the graph is aligned with that of the visible spectrum. This should help to determine the absorption features and clarify the relationship between both instruments.
For those who have a particular interest in the spectrophotometer I produced my first graphs several years ago, some of which I introduced on my web page THROUGH MY SPECTROSCOPE– see entries for November 2011; September 2012.’August 2016 & 2017. My plan is now to display others on the principal gemstone page with the visible spectrum.
The majority of the gemstone spectra are from specimens in the authors private teaching collection. However some are from specimens kindly on loan from Alan Hodgkinson (specimens prefixed AH) and the late Dr. Jamie Nelson (specimens prefixed JBN.) My thanks go to them and all my colleagues and fellow gemmologists for their help and encouragement in compiling this database.
Copyright John S. Harris - from the year 2000 to the current date.
Copyright John S. Harris 2000-2015
SYN. BLUE SPINEL & Co. GLASS
Other pages are under construction