Dichroscopes reveal whether transparent, coloured gemstones exhibit pleochroism, i.e. direction-dependent polychromatism. Light beams that pass through such stones exhibit quite different characteristics, depending on which crystal system the stone in question belongs to. Some gemstones appear in different colours when viewed from different directions. Imitations frequently differ from natural stones because of this property.
Where do colours originate? Colours in gemstones only develop when they allow part of the light to pass through or reflect and absorb another part. A good example is the ruby: It acquires its striking red colour because it lets the red light and part of the blue light through or reflects it. The remaining colours of the visible spectrum are absorbed by the ruby.
The light emanating from the stone to be examined is broken down by a calcite crystal inside the dichroscope into two adjacent light beams polarised perpendicular to each other.
In our example, the green light from a green tourmaline falling into the dichroscope. Here it is separated into two beams: a north-south oscillating “extraordinary beam” and an east-west oscillating “ordinary beam”. Since the two beams were absorbed differently in the tourmaline due to their different oscillation directions, one appears in green and the other in a blue-green colour.
Do you want to read more?
If you are looking for more specialist information, you should order the white paper “Gemstone Dichroscopy”. The publication describes e.g. the following topics:
- Design and mode of operation of a dichroscope
- Colour origin of a citrine
- How does pleochroism develop?
- Practical tips: How to examine pleochroism correctly with a dichroscope
- Colour Chart and Analysing Sheet for Colour
- Identification Distinguishing between amethyst and glass with the dichroscope
- Identifying emeralds by their multicolouring (dichroism)
- Pleochroism in tourmaline
- Observe pleochroism (trichroism) in a tanzanite crystal
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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