by Ritika Changrani
Polariscope is an instrument used in gemmology which helps to find if the gemstone is single refractive or double refractive and also allows determining the various crystal axis of the stone.
A Polariscope is used to determine whether the gemstone is natural or synthetic. This is one of the most commonly used instruments by the gemmologists. The use of plane polarized light allows us to see the actual path the beams are taking through the stone.
A polariscope is made of two Polaroid plates that rotate, a power switch, a stone holder and a bottom light source. Two polarized filters or plates are made of polarizing plastic sheets, one is on the top of the instrument, known as analyzer and the other is on the bottom of the instrument, known as polarizer. Polarizer and analyzer have their own vibrational planes. When the vibrational plane of polarizer and the vibrational direction of the analyzer are at right angles to each other, the field between them appears dark. This position is known as crossed position and it is in this position that the gemstones are tested to see if they are isotropic, anisotropic, anomalously double refractive and anisotropic aggregate. The polariscope can be used to determine the optical character as well as the optical sign of the gemstone.
Polariscope can also be used to determine the strains in diamond. It helps in separating natural gemstones from synthetic ones and also helps to distinguish solid inclusions from negative inclusions. Polariscope can also be used for recognizing polysynthetic twinning.
How to use a Polariscope?
- The polariscope is adjusted for cross position by setting the vibrational plane of polarizer and the vibrational direction of analyzer at right angles to each other.
- Turn on the light source and place the gemstone on the rotating platform above the polarizer. The gemstone is rotated in a vertical axis.
- Observe the gemstone through the analyzer and note down the observations
- Based on the observations the nature of the gemstone can be determined as follows.
- If the stone remains dark throughout the 360 degree rotation, the stone is said to be isotropic or singly refractive.
- If the stone darkens evenly at every 90 degree interval, the stone is said to be doubly refractive or anisotropic.
- If the stone appears light throughout the rotation, it is said to be microcrystalline or crypto crystalline aggregate.
- If the stone shows anomalous double refraction, the stone is singly refractive. Anomalous double refraction may be confusing at times. Hence when there is a suspicion that about anomalous double refraction, it is recommended to orientate the stone in the position where it appears lightest. Then turn the analyzer to 90 degree. If the stone appears lighter than before it is said to exhibit anomalous double refraction and hence is singly refractive. If the stone appears the same, it is doubly refractive.
How does a Polariscope work?
Polariscope works on the principle of plane polarized light. Single refractive gemstones do not break the plane polarized light into various colours. There is only one beam of light coming out. Examples of singly refractive gemstones include diamonds. Double refractive gemstones break the plane polarized light into two paths. One is the ordinary beam and the other is the extraordinary beam. Based on analyzing the transmitted light, the nature of gemstone can be identified. Some of the doubly refractive gemstones are quartz, tourmaline, ruby, zircon, peridot, sapphire and rulite.
Both the filters i.e. polarizer and the analyzer are turned to the dark position. When plane polarized light is passed through the gemstone, the singly refractive stone remains dark since it does not change the path of light. Hence it remains dark when observed in all positions. But in case of double refractive gemstone, when plane polarized light is passed through it, the stone changes the path of the light and hence the direction of the light wave changes. This produces light that is no more polarized. The change in direction of light makes the stone change from light to dark as it is rotated between the Polaroid plates.
Since the polariscope works through plane polarized light, it can be used to test transparent as well as translucent gemstones, but cannot be used to test opaque materials. Once the stone is found to be doubly refractive, a conoscope can help to determine the optic interference figure. Optic interference figure helps to locate the various directions in which the light is traveling through the stone. This information in turn helps to identify the type of gemstone. The different shadow patterns observed through the polariscope helps to determine the crystal structure and diagnostic patterns of the gemstone.
Types of Polariscope
There are two types of polariscope i.e. plane polariscope and circular polariscope. Circular polariscope work on circular polarized light unlike plane polariscope which works on plane polarized light. In a circular polariscope, two quarter wave plates are added to plane polariscope. One quarter wave plate is placed between the polarizer and the gemstone to be tested and the second quarter wave plate is placed between the analyzer and the gemstone. The quarter wave plates produce circularly polarized light. The advantage of using circular polariscope is that it helps distinguish isochromatics and isoclinics. But plane polariscope are more extensively used for testing gemstones.