Refractometer

This gemological instrument is based on the principle of "total internal reflection".

Uses of refractometer:

  • To determine Refractive Index.
  • S.R. or D.R.
  • Birefringence
  • Optic Character (Uniaxial or Biaxial)
  • Optic Sign (Positive or Negative)
  • Optic Axis Direction
  • Approximate R.I. on curved surfaces.

Important to Note:

  • The R.I. can be taken on any stone that has flat or curved surfaces as long as the surface is polished.
  • No rough gemstone should be tested on the refractometre.

A general procedure is explained for taking R.I. measurements. There are two distinct types of readings taken for flat facets (flat facet R.I.) and for curved surfaces such as cabochons (spot R.I. or distant vision method).

Procedure:

  • Clean the stone.
  • Place a very small drop of contact liquid onto the glass prism.
  • In facetted stones use the largest well polished facet, for cabochons or curved surfaces use the surface which has the best polish.
  • Gently slide the stone onto the contact liquid with your fingers. Do not use tweezers.
  • Under white light / monochromatic light, observe the image of the stone on the scale as a black spot having the shape of the contact facet.
  • The spot will correspond to the shape of the facet, i.e. it may be round, oval, square or octagonal, as the case may be.
  • Move your head up and down while observing the black spot:
    • For cabochons, curved surfaces, very small facets and for poorly polished facets, the reading is taken where the spot is
      • Exactly half light and half dark, or
      • The spot blinks from dark to light, or
      • If the spot lightens gradually, take the reading where the spot changes to a completely light spot i.e. the last all dark and first all light spot.
    • In the case of cabochons etc. with high birefringence, the D.R. can be obtained by using the birefringence blink method. While observing the spot place the polaroid filterover the eyepiece and note the reaction.
      • The spot will blink from light to dark, or
      • The spot will blink from red to green.
      • Note: The reading where the spot starts blinking at the top (lower reading) and then note the reading where it stops blinking (higher reading). Take both the readings and the difference between these two readings will give the birefringence.
  • For exact and precise R.I. measurements on flat facets, the R.I. will appear as a sharp grey / black line or cut off.
  • Place the polaroid filter over the magnifier and rotate. If the grey / black cutoff or line remains constant, then the stone is S.R.
  • If on rotating the polaroid filters the grey / black line moves up or down, then it is a D.R. stone and you have to determine the birefringence.
  • To determine the birefringenceof the stone:
    • Rotate the polaroid filter and note any shift in the shadow.
    • Take the readings of the stone from at least 4 different directions.
    • The difference between the maximum and minimum readings will give the birefringence of the stone.
  • Along the optic axis direction the stone will give only one single reading.
  • Over range stones will show either the grey cutoff reading or sometimes spectral colours at 1.80 while a spot reading will change completely white at 1.80 indicating that the stone has a reading over the range of the refractometer.
  • In the case of garnet-glass doublets, a red reflection will be observed when the reading is taken on the garnet surface, using white light. The base of the spot will reflect this colour which is known as the red - flag effect.
  • After taking the readings gently slide the stone from the glass prism and wipe the glass hemi-cylinder with soft tissue.

 

  • The glass prism has a very low hardness and hence will scratch easily. Be very careful while placing or removing the stone from the glass prism.
  • Care should be taken when testing mounted stones, so that the metal does not scratch the glass. Never use tweezers.
  • When not using the refractometer for long periods of time, place a thin film of vaseline to prevent tarnishing.
  • If the contact liquid evaporates and hardens to give sulphur crystals - Do not try to forcibly wipe the glass as it will scratch the surface of the prism. First remove the crystals with more contact liquid or M.I. liquid and then wipe the glass gently.
  • Evaporation of the liquid will tend to change its refractive index and thereby change the higher limit of the refractometer.
  • Good quality R.I. reading depends on the quality of polish of the stone and flatness of the facet.
  • Too much contact liquid will cause a dark ring around the stone that will confuse the readings. To avoid this use a very small drop of contact liquid.
  • Wipe the glass prism with soft tissue, in one direction only.
  • A Parallax error is basically an optical distortion. The readings tend to vary slightly between divisions. Care should be taken not to mistake this for double refraction.

Limitations:

  • Rough gemstones cannot be tested on refractometro.
  • Stones with R.I. more than 1.80 cannot be tested on refractometre.

Types of Refractometer

  • There are a number of different types of refractometers - Duplex (GIA model), Rayner, Eickhorst (some with LED source), RosGem models etc.
  • Most of the refractometers available are the critical angle refractometers.
  • Some refractometers employ a high R.I. prism made of strontium titanate or synthetic cubic zirconia, but the use of toxic contact liquids is a deterrent.
  • One of the newest refractometer on the market is the Brewster angle refractometer with a much higher range from 1.40 to 3.50.This instrument does not use a contact liquid and has the advantage of a wider range.

Refractometry

Methods of R.I. Measurement:
Immersion methods: A number of variations are in use. A few are briefly explained below.

  1. Becke Line Method: This method has been adapted by Mittchell and is used for facetted stones.
    • Requires a microscope with light field illumination.
    • The stone is placed in an immersion cell, table facet down.
    • The liquid used should be one whose R.I. is known.
    • Restrict the transmitted light to the area of the stone by adjusting the aperture below.
    • Focus of the microscope in the liquid above the stone.
    • If the facet edges appear black and then change to white as the focus is lowered into the stone, then the liquid has a higher refractive index.
    • If the stone has a higher R.I., the reverse would occur.
    • By using liquids of different R.l.'s a fairly accurate R.I. of the stone can be obtained.
  2. Immersion Contrast: This method observes the relief and appearance of the girdle and the facet edges when immersed in a liquid of known R.I.
    • Place the stones to be checked in a glass cell containing the liquid, table facet down.
    • Place the dish on a mirror so as to view the exact reaction while using a single light source from directly above.
    • When the liquid has a higher R.I., the girdle appears white, while the facet edges appear as black lines.
    • When the liquid has a lower R.I., the girdle appears black, while the facet edges appear white.
    • When the liquid and the stone have approximately the same R.I., the facet edges become almost invisible.
    • The thickness or width of the girdle edge can aid in determining the difference in R.I. between the stone and the liquid.
  3. Direct Measurement Method: This method requires a microscope with a vernier scale fitted on it or a calibrated focus adjustment.
    • Two parallel facets / surfaces are required i.e. the table facet and the culet.
    • Focus on the table facet and take a reading (A).
    • Focus through the stone till the culet is sharply defined and take a reading (B).
    • Focus on the glass slide on which the culet point rests, for an exact measurement of the culet (C).
    • Calculate the real depth (C − A) and the apparent depth (B − A).
    • The R.I.is calculated by dividing the real depth from the apparent depth.
      • real depth ÷ apparent depth = R.I.
    • This method is more effective for singly refracting stones, uniaxial stones (ordinary ray direction) and high R.I. stones.
  4. Minimum Deviation Method: This method uses a table spectrometer (goniometer) and is the most accurate method of measuring the refractive index. It requires both skill and ideal conditions for the results to be accurate.