Published: June 2013
For those who have already mastered the techniques covered in How-To: GemVue™ Table Top Polariscope”, this article is for you. While the polariscope is limited in the tests it can perform, there are certain cases where it is extremely helpful, particularly for transparent rough stones, cabochons, and beads.
When resolving the optic figure the stone must be:
-Transparent or nearly transparent
-Single crystal (not an aggregate)
-Of a size to fit in your instrument
1. Begin the test with the polariscope in the “dark position”, also known as “crossed filters”.
2. Hold the stone in your fingers between the crossed polars, then start rotating the stone in all directions to look for rainbow hues (interference colors) on the surface of the stone that will show you the optic axis. The optic axis follows an invisible, straight line through the stone.
3. Look for the figure using the follow-the-brush-technique. Find the blinking extinction pattern. Tilt the stone to look down the narrow end of the dark extinction brush. NOTE: For many biaxial stones, this is the only way to find the optic axis. In some stones it is extremely difficult to find and sometimes not possible.
4. If you don’t see interference colors or a dark extinction brush:
-Touch the optic figure sphere to all parts of the stone while you turn it. This might help you find an optic figure, even if you didn’t see a sign of an optic axis direction earlier.
-Place a drop of water on the optic figure sphere, or put the stone in an immersion cell filled with water. Liquid reduces surface reflection and internal refraction.
5. Once you find the interference colors, place the optic figure sphere on the heaviest concentration of intense colors. NOTE: Once you find interference colors in one direction you may try turning the stone 180°, then look for them in the opposite direction to see a clearer optic figure. * Let me know if you would like an image to show the interference colors
6.If the stone is cabochon cut or a bead, you will not need to do anything further to see the optic figure, as they serve as their own optic figure sphere.
If this is the pattern you see, it indicates the stone is uniaxial. As a single test, it will not identify the gem, but will eliminate many possibilities.
If this is the pattern, the stone is also uniaxial, but when the center is open rather than a solid black cross, it means it is quartz. Quartz is the only gem material that will show this pattern, but not all quartz. Some quartz will show a regular uniaxial optic figure. A bull's eye optic figure will indicate that the stone is quartz, but not whether it is natural or synthetic.
If any of these are the pattern seen, the stone is biaxial. Results vary widely as these materials have two optic axes and the exact orientation can give a variety of results. If not easily-seen, try turning the stone 180°. A biaxial optic figure by itself will not identify the gem, but will eliminate many possibilities.
Perform this test when you resolve only a partial optic figure (one-half of a biaxial or one-forth of a uniaxial figure, for example), as in the case of a highly fractured stone.
1. Find the partial optic figure under crossed polarizing filters.
2. Rotate the stone about its optic axis. NOTE: Be sure you rotate the stone while keeping the optic axis aligned in the same direction. Examine the stone carefully.
3. Interpret your test results:
-If the brush moves in the opposite direction of the rotation, the stone is biaxial
-If the brush doesn’t move, the stone is uniaxialPrecautions and Limitations
-Not all gemstones will reveal their optic axis.
-Not all gemstones will show an optic figure.
-For reliable test results, look parallel to the optic axis direction.
-It can be difficult to distinguish between a stone with a uniaxial figure and a biaxial stone with a double brush biaxial figure because the biaxial stone’s optic axes intersect at a small angle.
-Some uniaxial figures tend to distort
-Heavily included stones can cause distorted results as the light can reflect of inclusions.
-Facet patterns and crystal orientation may distort results.
-Always back up polariscope results with other tests to confirm identification, except in the case of quartz if you get a bull's eye optic figure.
When not using your GemVue Table-Top Polariscope please take the following precautions to extend the life of this product.
- Unplug the unit when not in use.
- Do not touch the lenses. Clean with a damp (water) lint-free cloth.
- Use the cover shown to protect the unit from dust.
- Store in a cool, dry place.
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