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Advanced

Polarizing microscope

Gemological microscope with a square 50mm polarizer below the immersion cell and a 25mm round analyzer held between tweezers. A

cheap alternative to a polariscope with the extra benifit of magnification.

Photo courtesy of John Huff, Gemcollections.com

With the aid of a few polarizing sheets one can turn the gemological microscope into a polarizing microscope for less

than USD 30.00.

Simply lay one of the sheets over the transmitting lightsource and tape the other one in crossed position below the

optics (or find your own way of doing something similar).

This enables us to distinguish between solid and negative crystal inclusions and many other internal features agemstone might have.

Alot of the following discussion involves such a setup, although most of it can be achieved with the usual gemological

polariscope aswell.

Partially healed fracture (fingerprint) in a pink

sapphire.

The same fracture in a pink sapphire

between crossed polars.

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Photos courtesy of John Huff, gemcollections.com

Honey On page 64 of Ruby & Sapphire (1997), Richard Hughes shows a Thai/Cambodian sapphire with tiny

droplets of methylene iodide to show the uniaxial interference pattern. The small drops act like tiny

conoscopes and when applied correctly, you will have an extra hand free (the one that usually holds the

conoscope). Hughes also suggested, through personal communication, the use of a small droplet of

honey which works very well.

This technique works best with magnification as the obtained figures are very small.

Monochromatic light 

With monochromatic lighting (such as a yellow sodium filter of your refractometer), the interference

pattern may stand out more clearly.

Poor mans polarizing microscope 

Buy two polarizing sheets (50 x 50 mm) and tape them in crossed position on your microscope. One just

above the lightsource and the other just below the optics. This setup will give you a polariscope with the

great benefit of magnification and you will find interference figures much easier to interpret.

Such a setup should not cost you more than USD 30.00 (probably less) if you already have a suitable

gemological microscope.

Lucky womans retardation plate 

Florists usually have cellophane plastic in which they wrap their flowers. This cellophane may work as a

quarter wave plate. The length of the roll is the fast ray, the cutting edge is the direction of the slow ray.

Simply cut a small piece from it and lay it on the polarizer (between the lower polarizing sheet and thestone) after you found the interference figure (at a 45° angle to the polarizer directions of course).

Other cellophanes like ScotchTape may also work.

Immersion 

Sometimes it is very hard to find interference figures. Place the stone is a shallow dish of water (or baby

oil) and rotate the stone slowly in it. You will find the interference flashes more easily in certain

circumstances.

The dish should be placed between the crossed polars.

Polariscopes are a very useful, simple and inexpensive to make piece of gemological equipment. They

are used to tell glass from gem materials synthetic spinel from all other materials, singly refractive

from doubly refractive, crystaline from cryptocrystaline material, doublets and triplets from other

gems, identify yellow Verneuil corundum (Plato test-see Liddicoat -GIA), determine quartz definitively

from other materials, tell whether a transparent gemstone is biaxial or uniaxial in its crystal system.

This is pretty good or equipment that may be as simple as a camera lens and a pair of polaroid®

sunglasses.

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Anomalous double refraction may occur in singly refractive materials with internal strains such as diamond, garnet,

synthetic spinel, amber, plastic, opal and glass. The extinction patterns here however do not occur at precisely 90o

intervals although rarely they may be close. Plastic and amber may show bright interference colours. Glass may

show a characteristic cross like shadow, or two approaching bars which almost form a cross during rotation.

Synthetic spinel shows a characteristic 'tabby' extinction, a sort of fine, mottled cross hatching of parallel silk-like

lines which change during rotation. In a doubly refractive stone interference colours will appear when one is within

a few degrees of an optic axis. To produce diagnostic interference figures one obtains the optic axis position and

placed a condensing lens above the stone or in contact with it. This can be a 10X lens (viewed from 18" or so

distance), a drop of viscous liquid or a strainless glass sphere. This will divulge whether the stone is uniaxial or

biaxial.

It may be noted that cabachon gems or beads often need no condensing lens as they function as one

themselves. Interference figures may be resolved more easily if the gem is immersed in water or

bromoform (toxic). Be sure that the transparent container for your liquid does not itself add shadow

lines to your image, especially if merely testing for double refraction. Interference figures may be used

to distinguish between: moonstone (orthoclase) and chalcedonic quartz which shows no figure

resolution, topaz and tourmaline, andalusite and tourmaline, corundum and chrysoberyl.

Tension in Diamonds

Tension of a diamond is also known as 'stress'. Tension can be identified

using polariscope through which light appears iridescent inside diamond.

Natural Characteristics of individual diamond related to internal structure are

visible or non visible inclusions. Presence of tension can cause diamond to

shatter.

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As viewed by a Polariscope - Tension in a diamond: