The Polariscope and its uses

The Polariscope 

Polariscope is a quick and useful tool that will help us to identify the optical nature of a gemstone and conclude whether it is singly or doubly refractive. These gems are known as isotropic or anisotropic respectively. This tool will also identify gems with a polycrystalline nature and on a few occasions, the polariscope can fully identify the gem.

 

A polariscope can be used on rough, cut, set, and loose gemstones. The only limitation is that the gemstone must have some transparency and that the setting does not inhibit us from viewing any light traveling through the stone.

There are two types of polariscope

·       A portable polariscope, which requires an additional light source

·       A bench (Table) polariscope with a built-in light source, which plugs straight into the mains.

A polariscope is made up of two polarizing filters.

 In the portable polariscope, these filters are fixed at a 90-degree angle to one another. To use the portable polariscope, open the polariscope fully by moving the two filters back on themselves to lock it into place. Put the polariscope on your light source and turn the light source on. The test results will be most easily seen in an area without surrounding light. When looking through the polariscope, the bottom stage will appear dark. This indicates the polarizing filters are at 90 degrees to one another, which is also known as the crossed position.

 In a bench polariscope, the top filter is rotatable and must be turned to be fixed at 90 degrees. With a bench polariscope, turn the top filter until the bottom stage appears its darkest.

 

When a gemstone is placed in between these two filters and turned, it will create one of four basic patterns of light and dark depending on the optical nature of the gem. This piece of equipment does require practice at first to ensure that you can recognize the patterns effectively.

1.       If the gemstone appears dark on one full 360-degree rotation, the gemstone is singly refractive and will either be cubic or amorphous. Gems that show this pattern include garnets, spinel, diamond, and glass.

2.       If a gemstone is a mix of light and dark throughout the entire 360-degree rotation, this is another test result for singly refractive gems, so either cubic or amorphous, but these gems also contain strain within its structure. This is known as anomalous extinction effects (AEE) or anomalous double refraction (ADR).

3.       If the gemstone appears light then dark four times in one full 360-degree rotation, the gemstone is doubly refractive and will be in one of the six doubly refractive crystal systems. Please note that the gem may not go dark all at once. The darkness may pan across the stone as it is rotated, but it will go completely light at four points during this rotation. Gems that show this pattern include corundum, topaz, zircon.

Noted:  iridescence is seen when looking through the polariscope, this indicates that you are looking along the optic axis of a doubly refractive gem. To be sure that this is the case and that you're not viewing iridescence in a fracture or other inclusion, turn the gem on a different axis and test again to see the light and dark patterns.

4.       If a gemstone appears light on one full 360-degree rotation, the gemstone is polycrystalline. Gems that show this pattern include polycrystalline quartz such as agate, aventurine quartz, and chalcedony. and beryl.