Polariscope

Before starting with polariscope there is need to understand polarisation.

Polarisation: In ordinary light which comes from a source such as the sun or a lamp, the wave motion is not confined to a single plane. Instead there are many waves vibrating in all planes normal to the line of transmission. This is known as unpolarised light. When this vibratory motion is restricted to a single plane, at right angles to the line of transmission, it is said to be polarised light or plane polarised light.

  • This can be better understood with the help of a polarising filter.
  • Light which is reflected off a polished surface is mostly polarised with its vibration direction being parallel to the reflecting plane.
  • If the polarizer is aligned such that its vibration direction coincides with the vibration direction of the reflected light, then the light will pass through the filter and will be visible to the observer.
  • When two polarisers are placed one over the other such that the vibration directionsn are at right angles to each other, they are said to be "crossed polars" or in " extinction position" and no light comes through them.
  • Gemstones can be analysed for their optic character by rotating them between the crossed polars and observing their reaction.

Polariscope: It is an instrument which consists of two polaroid plates mounted apart. The lower plate is generally fixed and is known as the polariser, while the upper plate can be rotated and is known as the analyser.

Uses of Polariscope

  1. To find the optic character
      • S.R. = Singly Refracting
      • A.D.R. (S.R.) = Anomalous Double Refraction (seen in S.R. stones)
      • D.R. = Doubly Refracting
      • A.G.G. = Aggregate

    Procedure:

    • Place the stone on the polariser. Begin with the polariscope in the Dark or extinction position. On looking through the analyser rotate the stone 360° with your fingers and note the reaction of the stone.

    Test the stone in at least 3 directions. If the stone:

    • Remains completely dark during rotation, it is S.R.
    • Remains completely light during rotation, it is A.G.G.
    • Shows distinct, snake-like bands or a patchy effect it is A.D.R.
    • Blinks from dark to light during rotation, it indicates D.R. or A.D.R. (S.R.) needs to be confirmed.

    Stones showing S.R. or A.G.G. need not to be confirmed whereas stones showing D.R. or A.D.R. need a confirmatory test.

    Confirmatory Test: In order to confirm D.R. or A.D.R. (S.R.):

    • With the polariscope still in dark position, rotate the stone to the position in which it appears the lightest, or observe one small area only which appears the lightest.
    • While observing only this lightest area of the stone, quickly rotate the analyser to the light position of the polariscope and not the reaction.
    • Eliminate external light effects with your fingers which can be used to mask most of the polariser while confirming the reaction.
    • If the light portion of the stone becomes:
      • noticeably lighter / brighter i.e. it transmits more light; it is A.D.R. (S.R.)
      • darker or remains the same, it is D.R.
  2. To locate the optic figure (uniaxial / biaxial): In order to resolve the optic figure, stones must be: doubly refracting, transparent and should be a single crystal and not an aggregate.

    Procedure:

    • Begin with the polariscope in the dark position.
    • Hold the stone in your finger and rotate it.
    • Look for interference colours.
    • If the colours are located, place the optic glass sphere /10x lens, in the area where you observe the heaviest concentration of the interference colours.
    • The colours when located may also be seen by turning the stone 180° and viewing from the opposite direction. This will also indicate the optic axis direction through the stone.
    • If you observe two black arms making a cross like figure with circular rings, or a bull's eye figure, it confirms that the stone is a uniaxial stone.
    • If you observe a bull's eye figure, the stone under test can be identified as quartz.
    • If you observe a single black arm, which is narrow at the center, it confirms that the stone is a biaxial stone.
  3. To detect pleochroism:
    • Begin with the polariscope in the Light position and rotate the stone on the polariser.
    • Observe one pleochroic colour, continue rotating till another pleochroic colour is seen.
    • When oriented properly, the colour observed will switch upon every 90° turn of the stone.
    • Pleochroic colours are not seen side by side as with the dichroscope.
    • The stone should be checked in all directions in order that all principal rotation directions have been analysed.

Limitations:

  • The test is applicable only for transparent and semi-transparent stones and not opaque stones.
  • Materials tested should be a single crystal and not an aggregate for detecting pleochroism.

Precautions:

  • Analyse the stones in different directions in order to observe the principal colours, since along the optic axes directions the stone will appear S.R.
  • Highly included or fractured stones will reduce transparency and can cause a stone to exhibit almost any reaction. A stone which may be S.R. or D.R. may give an A.G.G. reaction.
  • The reaction of small stones may be difficult to observe and interpret.
  • Red, orange and violet stones must be reconfirmed with the dichroscope as they tend to give wrong results, e.g. almandine garnet.
  • Some S.R. stones can exhibit almost any reaction, mostly due to internal strain e.g. hessonite garnet, red spinel.
  • An A.G.G. reaction may be seen in both S.R. and D.R. stones.
  • Some doubly refractive, uniaxial / biaxial stones, exhibit biaxial / uniaxial optic figures i.e. pseudo-biaxial and pseudo-uniaxial. Such stones need to be confirmed with a dichroscope.
  • Stones with higher R.I. and higher dispersion, tend to give misleading results, e.g. synthetic cubic zirconia must be examined in different directions.