Refractometer

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The refractometer is one of the most important tools in a gemological laboratory. It indicates (not measures) the refraction index of a gemstone, which often gives vital clues to the identity of a gemstone.

Although one would expect a refractometer to measure the refraction of a gemstone, this is not the case. Instead it is based on a unique optical phenomenon named Total Internal Reflection (or TIR).

For a better understanding of the refractometer, you should first need to understand refraction.

Basic

Construction of a gemological refractometer

Crosssection of a standard gemological refractometer
(Eickhorst SR 0.005)


Light (1) enters through the rear of the refractometer through an opening (1a) in (or before) which a yellow sodium filter can be placed. It then hits a mirror (2) which transmits the light to the center of the hemicylinder (3).
This hemicylinder is made of high refractive glass (usually LaSF-35 by Schott with a refractive index of ~ 1.88 at nD and a hardness of about 6.5 on Moh's scale).
At the boundary between the hemicylinder and the gemstone (4), the light will be partially refracted inside the stone and partially reflected in the hemicylinder (see below on Total Internal reflection). The reflected rays (5) will pass through a reading scale (6) and a lens (7) or a series of lenses, depending on the type of refractometer.
The reflected rays hit a mirror (8) which directs the light to the ocular (9) and then outside the refractometer to your eye (11).
The ocular (9) can slide in and out for better focus and is usually accompanied with a detachable polarzing filter (10).

As the hemicylinder has a relative low hardness compared to most gemstones, care must be taken to not scratch it. That would ruine your refractometer as optical contact between the gemstone and the cylinder will be impossible and will give you false readings.


Total Internal Reflection

Inside the refractometer: Total Internal Reflection

When light travels from an optically denser material (with higher index of refraction) to an optically rarer material (with lower index of refraction), all light that reaches the boundary of the two materials will be either reflected inside the denser material or refracted into the rarer material, depending on the angle of incidence of the light.

For every two media in contact, the dividing line where either the ray of light is reflected or refracted is fixed and can be calculated. This dividing line is named the critical angle (ca). On the left you find an image showing the critical angle as the red line.
When light reaches the boundary of the two materials at an angle larger than this critical angle (the blue line), the ray of light will be totally reflected back into the denser material. Light reaching the boundary at an angle smaller than the critical angle will be refracted out of the denser medium, into the rarer medium (the green line). All light travelling precisly on the critical angle will follow the path of the boundary between the two materials.

The standard gemological refractometer can make use of this phenomenon because the reflected rays of light will appear light on the scale, whilst the refracted rays are not visible (therefor appear black). The light/dark boundary shown on the scale of the refractometer is a visible representation of the critical angle.

N.B.: in this example the light seems to come from 3 light sources, but the principle is the same when coming from a single point.

A hemicylinder is used so there will be no refraction on the light entering nor leaving the denser material. Refraction doesn't occur when a light ray is at 90 degrees to the boundary (in a hemicylinder the incident and exiting ray always reach the boundary at a 90 degree angle when directed to the center).

Lightning

Proper lightning is one of the key features when using the refractometer.

Although one can get results using a white light source, the standard is monochromatic yellow light with a wavelength of about 589.3nm. This lightsource is historically used as it was easily (and a low cost) produced by burning table salt in a candle.
All gemological refraction indices are based on the use of sodium light (or nD). For more information see Fraunhofer.
The use of different wavelengths can produce different readings and as the refractive indices of gemstones are measured with an accuracy of 0.001 decimal, sodium light should be used. All gemological tables of refractive indices are produced using this light unless otherwise stated.

White light can be used for single refractive gemstones or to obtain a first impression. One should look for the boundary between the green and the yellow of the allochromatic white light source.
However for double refractive gemstones one should then switch to a sodium light source, simply for the reason that the double refraction readings in white light may easily overlap and it will be impossible to get a correct reading. And of course the boundary between the lighter and darker areas is better defined, making the reading easier to take.

Always buy a refractometer with either a sodium filter or a sodium lightsource.

Contact liquids

Here things get a bit more complicated.

Contact liquids are used to create an optical contact between the hemicylinder and the gemstone. This is to prevent air from trapping between the facet of the stone and the hemicylinder. Which would ruine the Total Internal Reflection effect.

As this contact liquid also has it's own refractive index, there will be Total Internal Reflection between the hemicylinder and the liquid, before the light reaches the stone. This is very undesirable, but alas neccessary.
Therefor we need to ensure that the tiniest drop of liquid is used and the stone doesn't float on the liquid. Just enough to create a "thin film".

The result is obviously two Total Internal Reflection readings, one from the hemicylinder-liquid and the other from the liquid-stone boundary. That is the reason you will also see a faint reading near the higher index of the scale on the refractometer, which is the reading of the liquid.

The refractive index of the liquid sets the limit of which stones can be tested on the refractometer. Usually they have a refractive index of 1.79, whilst some have a refractive index of 1.81. You can not measure stones that have a RI higher than the liquid used. Stones with a higher RI than the liquid will give you a "negative reading".

Liquids with higher RI are available, but they are so toxic that they are only used in specially equiped laboratories. They would ofcourse also need a special hemicylinder which will be of higher RI than the liquid.

You should always shield your contact liquids from light (especially for the 1.81 type) and care should be taken not to let the liquids crystallize.

The chemical formulas are:

  • 1.79 - Saturated solution of sulphur and di-idiomethane
  • 1.81 - Saturated solution of sulphur, di-idiomethane and tetraidioethylene

Always wash your hands after you made physical contact with the liquids. Not only for the smell.

Use of the refractometer

As with every instrument, success depends on proper usage.

First you apply a very small drop of contact liquid on the center of the hemicylider of the refractometer, after which you place the stone you want to investigate table down next to the hemicylinder and with your fingernail slide it on the center of the hemicylinder. For an oval stone, length wise.
At this point the contact liquid will suck under the facet and provide for optical contact between the stone and the hemicylinder. Do not apply any pressure to the stone by pushing it down on the cylinder as that would damage the cylinder. Repairs are very costly.
Close the lit of the refractometer to shield the stone from any surrounding light and remove the polarizing filter (if it hasn't been already).

Now, with the lightsource in place, place your best eye (usually your right one) just before the ocular of the refractometer and find the dividing line between light and dark on the scale. For gemstone cut en-cabochon, the technique is slightly different (see the Distant vision method below).
You should position your eye so that you look at a straight angle to the ocular, this to prevent a "parallax error".
The best way to make sure your eye is in the right position is if you can see the whole scale (or most of it) without moving your eye.

If the scale seems blurry, you can slide the ocular in and out for better focus.

Now you can start taking your readings (explained below).
When you finished, slide the stone of the hemicylinder again and remove the stone with your fingers if possible.
It is important to keep the hemicylinder clean, so use a clean cloth or tissue to gently wipe any remainding contact liquid from the cylinder. This by gently and without any pressure making a North-South motion.

As mentioned above, the hemicylinder is made off a relatively low hardness glass and can easily scratch. So always make sure you keep abbrasive materials and sharp objects (like tweezers) away from the hemicylinder.

Look at the images below how to properly use the refractometer.

Click images to enlarge

N.B: Some people find it hard to get a small drop of liquid directly from the bottle. A different technique is to place a series of small drops (usually 2 or 3) next to the hemicylinder and place the stone on the smallest one. Then sliding the stone and liquid on the cylinder together.
Alternatively one can loose excess liquid from the liquid rod by making a few drops next to the hemicylinder and apply the remainder direct on the refractometers hemicylinder.
Whichever method one prefers will work.

1.544


We notate refractometer readings to a precision of 0.001 (one thousands). The refractometer scale has subdivison indicators to 0.01 (one hudreds). Between the two horizontal bars which indicate the 0.01, you will need to estimate the final precision.
In the image on the right you will see that the shadow edge is between the 1.54 and the 1.55 bars. Between these two values we need to find the last precision.
As it just above the middle, the last precision is 0.004. So the reading is 1.544 .

Estimating the last decimal needs some practise.

Some refreactometers, like the Eickhorst ones, have a more detailed division of the scales which makes taking a reading easier. Although with a little experience it is not needed.

Faceted gemstones

Following is the method used for facetted gemstones. En-cabochon and sphere cut gemstones require a somewhat different technique which is explained in the "distant vision" section.

When taking refractometer readings we usually start with the largest facet (in general the table facet). Place your stone in the starting position and close the lit of the refractometer and make sure the lightsource is on.

Position your eye infront of the ocular in a way so that it is at a straight angle with the refractometer scale. You will now most likely see a dark region at the top of the scale and a lighter region in the lower part.
When you have chosen a monochromatic sodium light source, there will be a sharp line between the two areas. That is what is named the "shadow edge". Or you will observe 2 less sharp "shadow edges".
Place the polarization filter on the ocular and, while looking at the scale, turn the polarizer 90 degrees left and right. You will observe either of two posibilities:

  1. only one shadow edge is seen
    • the stone is either isotrope or
    • you are looking down the optic axis
  2. you see the shadow edge move between two values on the scale
    • the stone is uniaxial or
    • the stone is biaxial


  • In the first case the shadow edge will remain constant during a 135 degree rotation of the stone.

For every reading, we take two measurements: one with the polarizing filter in North-South position and one with the polarizing filter in East-West position.

The readings in the images below indicate a single refractive (isotrope) stone with RI = 1.527, which is most likely Glass (if one finds a single refractive stone with an RI between 1.50 and 1.70, it is most likely glass).
One should at this point verify that the stone is indeed isotrope and that you are not taking the RI on the optic axis. Therefor search for another large facet on the crown or the girdle to take another reading.

Taking four sets of readings on a single refractive stone looks like overkill, which it is, take them anyway.


First reading Second reading Third reading Fourth reading
1.527
1.527
1.527
1.527
1.527
1.527
1.527
1.527
  • In the second case you write down both values you see in table form below eachother.

Below are 4 sets of readings of a double refractive stone with a uniaxial optic character (one reading remains constant).
For every set of readings, you rotate the stone 45 degrees with your fingers without applying pressure and leaving the stone in contact with the hemicylinder.


First reading Second reading Third reading Fourth reading
1.544 ο
1.553 ω
1.544 ο
1.552
1.544 ο
1.549
1.544 ο
1.552
1st 2nd 3rd 4th
lower readings ο 1.544 1.544 1.544 1.544
higher readings 1.553 1.552 1.549 1.552


While taking your refractometer readings, you write down the values you read on the scale. For every set of readings the the polarization filter is turned 90 degrees. In addition to this you can also take a fifth reading (180 degree rotation).

In the example above the lower readings (1.544) stay constant and the higher readings vary.

Note: The lower reading is the reading of lower value, not lower on the scale.

The RI of this stone is 1.544 - 1.553 (smallest lower reading and largest higher reading). This indicates Quartz.
To calculate the birefringence of the gemstone you take the maximum difference between the largest higher reading and the smallest lower reading. In this example that is 1.553 - 1.544 = 0.009 .

  • Gemstones may also have two variable readings, the procedure remains the same.

You write down the lower and higher reading in a table and calculate the birefringence.


First reading Second reading Third reading Fourth reading
1.613
1.619
1.611 α
1.616
1.614
1.619
1.611 α
1.620 γ
1st 2nd 3rd 4th difference
lower readings; 1.613 1.611 1.614 1.611 0.003
higher readings 1.619 1.616 1.619 1.620 0.004


These readings give an biaxial reading with RI = 1.611-1.620 and a birefringence of 0.009, indicating Topaz.


Optical character

Spot readings (distant vision method)

Advanced

Optical sign

Bright line technique

Kerez effect

Different types of refractometers

Related Topics

Sources

  • Gemmology Third Edition - Peter G. Read
  • Gems, Their Sources, Descriptions and Identification 4th edition - Robert Webster, Anderson
  • Gem Identification Made Easy 3th edition - Bonanno, Antoinette Matlins
  • Gem-A Foundation and Diploma notes