Some gemstones can be distinguished by colour changes when viewed through special colour filters.
The Chelsea filter (coloured stones)and the ruby filter (red stones) are preferred for this gemstone examination. They are especially suitable for distinguishing different gemstones that are viewed together, such as for checking a gemstone lot. The stones can be rough or polished – the different colour reactions when viewed through the respective colour filter are decisive.
Function of colour filters for using it for examination a green emerald
The stones to be examined are placed on a white or black substrate and illuminated with an intense white light source. The assessment is then made by looking through the colour filters. The Chelsea filters; Ruby color filters are mostly colored glasses that let through one or more areas of the visible spectrum and filter out the rest of the light spectrum.
Our example shows a green emerald from Colombia (upper figure) that appears surprisingly brownish-red under the color filter (lower figure). This happens because the Chelsea filter only lets light through in the deep red range and in the yellow-green border range. Therefore it is almost only the red part of the emerald’s spectrum that reaches the eye of the viewer.
Do you want to read more?
If you are looking for more specialist information, order the white paper “Gemstone examination with colour filters”. The publication describes e.g. the following topics:
- Design and functioning of colour filters
- Emeralds analyzed under the chelsea colour filter
- Identifying synthetic emeralds with that colour filters
- Identifying aquamarine rough stones and counterfeits with the chelsea colour filter
- Analyse red or pink stones with the ruby filter
- Identifying composite stones like doublets or triplets with the colour filter
- Identifying chromium-rich stones under the ruby filter
The whitepaper was produced in collaboration with Prof. Dr. Jochen Schlueter wrote. He is a graduate mineralogist and responsible for the display collection of the Mineralogical Museum in Hamburg.
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