8
GIA Polariscope
9
GIA Polariscope
Precautions and Limitations
• The stone must be large enough for you to detect and interpret its reactions.
If the stone is very small, try placing the magnifier on top of the analyzer to
better observe the reaction.
• Never determine a transparent red stone to be doubly refractive until it has
been checked for pleochroism.
• Stones that are over the limits of a refractometer (OTL) can give indefinite
results in the polariscope. It’s best to confirm if the stone is singly or doubly
refractive by looking for pleochroism or the presence or lack of doubling in
a microscope.
Figure 1. At left, the gem is dark but turns light as the stone is rotated (right).
Locating Optic Axis and Optic Figure
Every doubly refractive stone cut from a single crystal has either one direction
(uniaxial) or two directions (biaxial) along which it is singly refractive, called its optic
axis. With the polariscope and an optic figure sphere, uniaxial and biaxial stones
display distinctly different characteristic patterns called optic figures.
To locate the optic axis and an optic figure:
1. Turn the analyzer to the crossed (dark) position.
2. Place the magnifier on top of the analyzer.
3. Hold the stone between the polarizer and the analyzer with your fingers.
4. Rotate the stone between your fingers as you look through the analyzer.
5. As an optic axis is approached, interference colors, which faintly resemble the
play-of-color in opal, should become visible. When the axis is exactly vertical,
these colors will be most pronounced. Strength of intensity of the interference
colors varies with the type of material being tested.
6. Place the optic figure sphere over, close to or touching the area with the brightest
interference colors to observe the optic figure (see figure 2). See typical patterns
of uniaxial and biaxial gems in figure 3, next page.
Interference colors are not always visible. In this case, rotate the stone horizontally
and look for a dark shadow, or “brush,” that sweeps across the stone when you move
it (see figure 4, next page). Touch the optic figure sphere to the narrowest part of the
brush to observe the optic figure.
Figure 2. Interference colors appear along an optic axis (right). Place the
optic figure sphere over the area with the brightest interference colors to observe
the optic figure.
