Polariscope

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The polariscope may be one of the most underestimated tools in gemology. Most gemologists use it to quickly determine if the stone at hand is isotropic or anisotropic or, at best, optic character of gemstones. With some small additions, one can determine both optic character and the optic sign of a gemstone. It is also the preferred tool -- next to the microscope -- for separating synthetic Amethyst from its natural counterparts (although with recent synthetics that may prove difficult).
In addition, the polariscope may be very useful for distinguishing solid inclusions from negative inclusions as well as spotting polysynthetic twinning.

Basic

The Polariscope

A polariscope uses polarized light for gem identification. It consists of two polarized filters, one on the top and one on the bottom of the instrument as seen in the picture to the right. Both the polarizer and the analyzer have their own vibrational planes. When the vibrational plane of the polarizer is at right angles to the vibrational direction of the analyzer, the field between them remains dark. This position is known as the "crossed position". In this position, gems can be tested to determine if they are:

  • Isotropic
  • Anisotropic
  • Anomalously Double Refractive or an
  • Anisotropic Aggregate

The polarizing filters of this instrument are made of a plastic with microscopically oriented crystals of iodoquinine sulfate.


Operation of the polariscope and possible observations

With the polarizer and analyzer in crossed position, turn on the light source and place the gemstone on the rotating platform just above the polarizer (this platform might not always be present, in which case you use your tweezers).
Observing the gemstone through the analyzer while slowly turning the stone will give you 4 possibilities.

1. The stone appears dark throughout a 360° rotation.

The stone is isotropic (single refractive).

2. Throughout a 360° rotation the stone blinks 4 times, light and dark.

The stone is anisotropic (double refractive).

3. The stone will appear light all the time.

The stone is a microcrystalline or cryptocrystalline aggregate (like, for instance, Chalcedony).

4. The stone will show anomalous double refraction (ADR).

It is isotropic (single refractive).


The first 3 behaviors should pose no problems for the inexperienced user, but the latter (ADR) can be misinterpreted and cause one to think the stone is double refractive.


Video.png Anomalous Double Refraction video
Video showing the behavior of double refractive stones and ADR under the polariscope - WMV/video format - 5480KB

A possible solution for overcoming the confusion when one suspects ADR is to orientate the stone in its lightest position and then quickly turn the analyzer 90°. If the stone becomes noticeably lighter, it means the gemstone is single refractive is exhibiting ADR. If it stays more or less the same, the stone is double refractive.

Red stones that are out of the limit of the refractometer (OTL) may be especially difficult to distinguish with the polariscope due to ADR. Some stones in this category are Ruby, red Spinel and red Garnets.

Note: It should be noted that the gem being examined should be transparent to translucent so light can pass through it. If you put a piece of floor tile under the polariscope it would remain dark, but that doesn't mean the tile is single refractive. It just means light can’t pass through it.

Anisotropic gemstones can have one direction or two in which it will stay dark throughout lateral rotation. These directions are the optic axes of the gemstone. Uniaxial stones have one optic axis, biaxial gemstones have two. No double refraction occurs along the directions of optic axes.
Because there may be more than one direction in which some gemstones remain dark, it is useful to view the stone under a different angle when it stays dark as a confirmation.

Advanced

Conoscopy

Conoscope.jpg

In gemology we use a conoscope (a strain free acrylic or glass sphere on a rod) to determine optic character (uniaxial or biaxial) in anisotropic gemstones. The conoscope creates a 2-dimensional image of the 3-dimensional interference in a mineral.
Although determining the optic character with a conoscope is a fairly easy procedure, finding the interference figure itself is not. The interference figures always appear around the optic axes of minerals.

The simplest way to find an interference figure is to rotate the stone under the polariscope, in every possible direction, while looking down the analyzer until one sees a small flash of colors appear on the surface of the gemstone. When that flash of colors is found, fix the stone in that position and hover your conoscope slightly over it. Now, while still looking through the analyzer, you should see the color flash transform into a rounded 2-dimensional image.
This image will appear different in uniaxial stones than in biaxial stones, each having its own characteristic pattern.

Using an immersion cell along with the polariscope may enable you to find the flash figures more rapidly.

Due to enantiomorphism , Quartz will give a typical uniaxial image but with a large "eye" in the middle. That is what is named a "bull's eye" and is typical for twinned Quartz (both natural and synthetic).

Because anisotropic minerals appear to be single refractive when viewed down the optic axis, another technique for finding the optic axis can be used. View the stone under the polariscope from all sides to find where the gemstone does not blink light and dark on lateral rotation. That will be the optic axis.
Remember that uniaxial minerals have one optic axis while biaxial gemstones have two optical axes.

The above typical images may not be seen as a whole or very sharply at times, but don't be alarmed. One can determine optic character from part of the conoscopic image.

Interference figure nomenclature

For clarity the nomenclature of interference figures should be understood. Luckily this is not too difficult.

Uniaxial


In uniaxial stones, the "melatope" indicates the center of the dark cross and is the direction of the optic axis (you are looking down the optic axis).
The dark cross is actually made up of two V-shaped "isogyres" that will always stay in the same position in uniaxial stones.
The colored concentric fringes are named "isochromes".

Biaxial


Biaxial minerals have two optic axes, hence they have two "melatopes" that are in the center or the isogyres.
Again the dark cross is made up of two brushes, named "isogyres".
The colored concentric fringes are named "isochromes".


Biaxial with 45° rotation


When the biaxial interference figure is laterally turned, the isogyres detach and transform into hyperboles. The maximum curvature of these hyperbolic isogyres is at 45° rotation.
The distance between the two melatopes is dependent on the "2V" value of the mineral. When this 2V value is large (about 40-50°), the two isogyres will rarely ever be seen in one image. This also depends on the "numerical aperture" of your microscope.
No knowledge of "2V" or "numerical aperture" is needed for our discussion.

Retardation

In mineralogy, retardation means that one refracted ray of light is lagging behind another ray of light.
When light enters an anisotropic (double refractive) gemstone, it is split into two rays -- a fast ray and a slow ray. Because the fast ray travels faster through the gemstone it will be ahead of the slow ray. When the slow ray leaves the gem, the fast ray would have already traveled an extra distance outside the gemstone. That extra distance is known as "retardation" and is measured in nm (nanometers).

Through a series of calculations, it is shown that this retardation is dependent on the thickness and birefringence of the gemstone.

When the stone is placed between two polarizing filters (a polariscope), the two rays combine at the analyzer and either interfere with each other or cancel each other out, depending upon whether the rays are in phase or out of phase, and the typical interference colors are seen.
These colors show a distinct pattern as seen in the Newton Color Scale below and, again, depend on the thickness and birefringence of the material.

I Newton Colors.jpg

When one would increase the thickness of the gemstone, the colors would shift to the right.
This knowledge can be useful in gemology as one could also add another mineral on top of the gemstone to mimic the increase of thickness and thus create a shift in colors when viewed through the conoscope. This shift can either be to the left or to the right.
When the slow ray of the gemstone and the slow ray of the added mineral align, the shift will be to the right. This will create an addition in color on the Newton Color Scale. When the slow ray of the gemstone and the fast ray of the added mineral align, the shift will be to the left and will create a subtraction in color.

When, for instance, a gemstone would create a retardation of 550nm, the starting spectrum would be on the boundary of the first order and second order and go from magenta to blue to blue-green to yellow to red. When one would add a mineral with a retardation of 137nm, the starting color would be blue (at 687nm) instead of magenta. (That is if the slow ray of the gemstone aligns with the slow ray of the added mineral).
When the slow ray of the gemstone would align with the fast ray of the added mineral, there would be a subtraction, so the starting color would be (550-137) 413nm. So, yellow-orange.

Retardation Plates

We however do not know what the slow ray nor the fast ray is in a particular gemstone or of the added mineral, so this knowledge is of little use with two uncertain variables. Therefore "retardation plates" are made.

Retardation plates (as those added minerals are known) have a known retardation and the orientation of the slow and fast rays are known which can help us determine optic sign in gemstones.

Mineralogists in general use 3 kinds of retardation plates:

  • quarter wave plates (with a retardation or 137nm) - made from mica
  • full wave plates (with a retardation of 550nm) - made from gypsum
  • Quartz wedges (with an increasing retardation of 0 to 550nm) - made from quartz

All of the above plates can be very expensive as they are usually designed for petrographical microscopes, which require special slots in the microscope. Fortunately, modern day technology created anisotropic plastics that can substitute them at low costs and can be held between your fingers. These can be used in conjunction with the standard polariscope or with an adapted gemological microscope where polarizing filters are placed just above the light source at the base and just below the optics.
The latter is a setup that transforms your microscope into a polarizing microscope for under USD 10.00 with the great benefit of magnification.

Those plastic retardation plates can be obtained from various sources (like the Daly/Hanneman simulated Quartz wedge from Hanneman Gemological Instruments) at low cost. If you are intent on buying a plate, make sure you know how the fast and slow rays are orientated. With a stone of known optic sign you can determine that yourself though.

Typical polariscope setup with conoscope and retardation plate

Determining optic sign with the use of retardation plates

Determining the optic sign in anisotropic gemstones should pose little problems with the aid of one of the retardation plates. The real challenge however is finding the interference figure.
All images below are conoscopic images (with the conoscope in place).

The plates should be placed directly under or directly above the gemstone. When above the gemstone, the plate is should be placed between the stone and the conoscope.

The plate itself should be inserted at a 45° angle to the polarizer and analyzer as illustrated in the images below.


Full wave plate on uniaxial stones
Conoscope uniaxial1.jpg


For convenience, the image at the left has the area of interest marked, which is the area just around the center of the interference figure (the white circle).
That area is divided into 4 quadrants.

Conoscope uniaxial5.jpg


The full wave plate is inserted from bottom right to top left at an angle of 45°.
In the direction marked "slow" the slow ray of the wave plate travels, the fast ray travels in the direction of the length of the plate.

When one looks closely (click the image for a clearer, larger view) the colors in the quadrants change.
Quadrants 1 and 3 turn more or less blue (here addition of color occurred), while in quadrants 2 and 4 the colors change to predominantly yellow-orange (here subtraction occurred).

This indicates a uniaxial gemstone with a negative optic sign.

Conoscope uniaxial2.jpg


The wave plate removed for clearer view (this is for illustration only and will not work in practice).

The quadrants 1 and 3 clearly have a shift of color to blue. Also notice that the dark cross (the isogyres) now have a magenta color. This is caused by the magenta color of the full wave plate under crossed polars (the color in natural daylight is transparent white).

Conoscope uniaxial4.jpg


The opposite of the above. Quadrants 1 and 3 show a yellow-orange color, while quadrants 2 and 4 turn blue.

This indicates a uniaxial stone with a positive optic sign.

Full wave plate on biaxial stones
Biaxial circle.jpg


The full wave plate is best operated on biaxial gemstones which are orientated in a way that the isogyres are at 45° to the polarizing filters.
We concentrate on the areas just around the melatopes, indicated by the white circle around the melatope in the top right isogyre.

As can be seen, the areas inside the circle (on the convex and concave sides of the isogyre) are more or less gray. These colors will change when a full wave plate is inserted.

In this image both the two isogyres are visible, not many gemstones will show this image.

Ideally, a biaxial gemstone will show both isogyres in one image, but alas that is not always the case.
As with the uniaxial stones, the wave plate is inserted at 45° to the polarizing filters (as illustrated).


Conoscope biaxial2.png Conoscope biaxial negative.png Conoscope biaxial positive.png
Rotation of 45° with isogyres at maximum curvature.
Colors on convex and concave sides are of 1st order gray.
Inserted full wave plate creates blue colors on the convex sides of the isogyres and yellow on the concave sides.

This indicates a negative optic sign.

The wave plate creates yellow 1st order colors on the convex sides and 2nd order blue around on the concave sides of the isogyres.

This means the stone has a positive optic sign.


Most of the times you will only see one of the isogyres at one time. When you do, rotate it to maximum curvature as seen in the images below.
When the 2V values are high (close to 90°), the curvature is almost impossible to recognize and you will have a hard time trying to see which direction it curves to (up or down). At other times the isogyre is a very fuzzy hyperbole which gives the same troubles. With some practice you will be able to recognize it more rapid.


Conoscope biaxial4.png
Conoscope biaxial3 negative.png
Conoscope biaxial3 positive.png
Same situation as above with the focus on one of the isogyres.
Without full wave plate inserted Optic sign is negative Optic sign is positive
Quarter wave plates and Quartz Wedges

Quarter wave plates work in a similar way as full wave plates but will produce different images.
With Quartz wedges the illusion of movement of the isochromes becomes important to determine optic sign.

Sources

  • Guide to Affordable Gemology (2001) - Dr. W. Wm. Hanneman, PH.D.
  • Introduction to Optical Mineralogy 3rd edition (2004) - Prof. William D. Nesse
  • Ruby & Sapphire (1997) - Richard W. Hughes
  • Gem Identification Made Easy 3rd edition (2006) - Antoinette Matlins, A.C. Bonanno

External links