How-To: Advanced Use of the GemVue Table Top Polariscope

Published: June 2013

For those who have already mastered the techniques covered in How-To: GemVue™ Table Top Polariscope,END REMOVED --> 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.

Resolving the Optic Figure:

When resolving the optic figure the stone must be:
-Doubly refractive
-Transparent or nearly transparent
-Single crystal (not an aggregate)
-Of a size to fit in your instrument

Resolution Test

1. Begin the test with the polariscope in the dark position, also known as crossed filters.

axis orientation

Possible Axis Orientation

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 dont 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 didnt 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.