When viewed through special color filters, some precious and gemstones can be distinguished by color changes.
The Chelsea filter (colored 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 decisive factor is the different color reactions when viewed through the respective color filter.
Function of colour filters for using it for examination a green emerald
The stones to be investigated are placed on a white or black background and illuminated with a strong, 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 through light in the deep red range and in the yellow-green border area. Therefore it is almost only the red part of the emerald’s spectrum that reaches the eye of the viewer.
Investigating red gemstones with the ruby filter
The ruby filter makes selected gemstones appear in a different color thanks to its specially colored glass. Under the ruby filter, only those stones that contain chromium glow an intense red. The ruby filter is also ideal for doublets or triplets and glass imitations quickly.
A particular highlight: synthetic emeralds from the flux process appear under the ruby filter in a strikingly intense red. This immediately raises the suspicion of a synthesis – and shows when further gemmological investigations make sense.
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More InformationRuby filter investigation in practice
The ruby filter is an extremely useful screening tool for the initial assessment of red gemstones. In his latest information video, gemmology influencer Dimitri Zlatev shows how the ruby filter from KRÜSS Gemmology is used professionally. Illumination is provided by the KRÜSS Optronic cold light sources.
All of Dimitri’s videos can also be seen on our YouTube channel.
Want to read more?
If you are looking for more specialist information, order the white paper “Gemstone examination with colour filters”. The following topics, for example, are described in the specialist publication:
- 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 white paper was written in collaboration with Prof. Dr. Jochen Schlüter. He is a qualified mineralogist and is responsible for the display collection of the Mineralogical Museum in Hamburg.
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