Dichroscope

by Ritika Changrani

Dichroscope is a tool used to differentiate similar looking gemstones based on the dichroic property of the gemstones.

Some gemstones are singly refractive. They exhibit only one colour to observer. There are also gemstones which exhibit two different colours of light. But most of that is not detected by the human eye since the two colours produced are so close in wavelength that it becomes difficult for the human eye to identify them. The two colours in such gemstones mix up to give a slightly different colour. Such stones are called doubly refractive gemstones. For example, ruby is made up of dark red and light pink colour. But it appears to the viewer as light red.

Dichroscope

The property of the gemstone to exhibit two different colours or shades when turned in two different directions under light is called as dichroism. Examples of the gemstones which are Dichroic are sapphire, zircon, tourmaline and topaz.

Dichroscope is a tool used to differentiate similar looking gemstones based on the Dichroic property of the gemstones. Hence it can easily differentiate gemstones like ruby from tourmaline since ruby is Dichroic and tourmaline is pleochroic. Dichroscope also helps to determine Pleochroism and trichorism. Dichroscope helps to determine the optical properties of a gemstone. (image courtesy: www.faceters.com)

Dichroscope is a metal tube which is open on one end and has a lens at the other. It has an optical calcite mounted inside the tube and produces a double image of the square opening. Enough bright light is directed on a gemstone and viewed through the dichroscope. Dichroscope separates the different colours of light into its constituent colours. Light entering the dichroscope is broken into two polarized rays. Two images of different colours indicate that the vibrations are of different wavelengths and at right angles to each other. If dichroism is not detected in the first test, the gemstone has to be turned and output colours must be viewed in other directions too.

Pleochroism can be detected by rotating the instrument at an angle of 90 degrees. If the two colours switch sides on the split image, the gemstone is pleochroic. To determine trichorism, it is necessary to change the orientation of the stone. If one new colour is determined along with one of the colours of previous orientation, the gemstone exhibits trichorism. Trichroic gems show three different colours when viewed in three directions perpendicular to each other. Singly refractive stones produce only one colour on the dichroscope.

How does a Dichroscope work?

Dichroscope works on the principle of plane polarized light. Plane polarized light is the light of a specific wavelength that is separated from ordinary light and travels in a specific direction. Some waves may travel up and down and some side to side. When light traveling in different direction is being separated, it is possible to view the individual colours of specific wavelength.

Types of Dichroscopes

There are two different types of dichroscopes. They are London dichroscope and Calcite dichroscope.

London dichroscope

In a London dichroscope, two pieces of Polaroid orientated at right angles to each other are placed such that the light entering each polarizing filter are at right angles to each other. The two polaroids in a London dichroscope are joined together in same plane. London dichroscopes are much cheaper when compared to the calcite dichroscope but does not give as clear a result as the other. It is difficult to determine gemstones with weak Pleochroism with a London dichroscope.

Calcite Dichroscope

The calcite dichroscope is made up of a calcite crystal. The calcite has a special characteristic called bi-refringent. This property of calcite enables the gemstone to appear doubled when looking through it. The calcite crystal splits the light entering the dichroscope into two rays and thereby helps identify the difference in colours. The two rays form two images at right angles to each other.

Out of the two dichroscopes, the calcite dichroscope is the most commonly used in the gem testing laboratories to test the Dichroic property of the gem stones.

How to use a Dichroscope?

Hold the upper portion of the dichroscope near the eye - may be an inch from your eye. Place the gemstone near the opening. The other end of the dichroscope is hexagonal and hence it is easier to turn the dichroscope while testing. View the gemstone through the gem view opening. Slowly rotate the dichroscope to complete one full rotation and observe the colour difference of the two small squares which appear inside the dichroscope while you rotate. If you observe two colours while you rotate, the gemstone is dichroic. If three colours are observed, the stone is trichroic. Repeat the observation to confirm the number of colours observed. Observe the strength of the colours as it helps determine the type of gemstone. Always make sure that sufficient bright light is focused on the gemstone while you observe the colours using dichroscope. Never use fluorescent light since it can give a false result.

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