Basic gemmological characterization was performed on 46 samples at HRD Antwerp, and included visual observations, RI and hydrostatic SG measurements, polariscope and
dichroscope reactions, hand-held spectroscope absorption spectra, and long- and short-wave UV fluorescence observations.
A device called a Hanneman Tanzanite Filter (Hanneman Gemological Instruments) is designed for detecting tanzanite simulants; it consists of a simple
dichroscope plus a Hanneman Aqua-Filter designed for characterizing blue gems.
An essential tool during the examination is a small, simple instrument called a
dichroscope. At one end is an eyepiece and at the other a small square opening.
With the
dichroscope, weak trichroism could be observed: colourless, light yellowish green and light green.
The identifications were performed on gem rough and cut stones through the application of gemological techniques (refractometer, polariscope,
dichroscope, microscope, heavy liquids, inclusion studies according to the techniques of Eduard Gubelin, etc.), using published data and identified examples in the collections of the author and the Ouro Preto School of Mines as standards for comparison (see Bastos, 1992).
'Pleochroism, the
Dichroscope and Colour Filters' then examines the range of colour filters available on the market today, along with the typical
dichroscope and Chelsea Colour Filter.