If there is one instrument that is undervalued in the field, it is polariscope. It is an easy to carry and robust instrument. Most of the time, after a magnifying glass, the first reflex is to take out the refractometer. The polariscope nevertheless makes it possible to distinguish isotropic stones from anisotropic stones, uniaxial gems from biaxial gems, but also to highlight particular reactions that are found for example in glasses, synthetic spinels and polycrystalline stones.
To understand how a polariscope works, you have to understand what light is made of. Light is made up of a set of different wavelengths (colors) that vibrate in different directions. This assembly constitutes a light beam. We can observe the decomposition of this beam by a prism.
When we talk about vibration in one direction, it refers to the wave shape that the light wave takes. Now imagine that we place behind a filter made up of a multitude of microscopic straight stripes. This is the polarizing filter. This filter would stop waves that are not in the same direction as the stripes. Here is an example of polarization:
It can be seen that the waves vibrate in all directions (the circle) and emerge from the filter in a vibration in only one direction (here horizontal). If we added another filter at the output of the first, but with the stripes perpendicular to those of the first, we would block the entire light beam. This is exactly what happens with a polariscope, which is composed of two polarizing filters crossed at 90 °! When we observe a light source through a polariscope, we do not see anything. Care must be taken to cross the filters at 90 ° if the filters are mobile. This position is called the extinction position. Note that you should always observe a gem from several possible angles so as not to draw a false conclusion if the stone is in its optical axis.
A stone placed between the two filters can either deflect the polarized light of the first filter, or not to deflect it. In the second case, the stone will appear very dark and it will be nice to rotate it on itself by varying the positions, it will remain dark. This stone is said to be isotropic, and it is characteristic of a gem of the cubic system. Conversely, a stone, generally opalescent and non-transparent, which remains lit on a 360 ° rotation in all positions will certainly be microscrystalline. This is due to the fact that it is made up of a multitude of small crystals that have grown in different directions and so there are always crystals oriented in a direction that allows light to pass through. Jades typically have this reaction.
The first indication that a polariscope can give is therefore to distinguish isotropic stones from anisotropic stones and microcrystalline stones. Isotropic stones are of the cubic system (see the crystallography part of the site) while anisotropic stones are part of the 6 other crystal systems.
There is still one use for the polariscope: to distinguish uniaxial gems from biaxial gems. For this, we use an additional instrument called the conoscope. It is a glass ball welded to the end of a rod. It makes it possible to materialize the optical axes which present a specific figure. It is often difficult to find these figures, but when we do, we are able to reduce the possibilities of the crystal system. Uniaxial stones are hexagonal, rhombohedral and quadratic systems. Biaxial stones necessarily belong to the orthorhombic, monoclinic or triclinic systems. Finding a cross figure indicates a uniaxial gem. If we see this figure, then a gem that we considered to be a tanzanite (orthorhombic) cannot ultimately be one.A particular figure is the bull’s eye which is diagnostic of quartz, it is a cross whose branches meet on a central green or mauve circle. Finally, a mustache-shaped figure indicates a biaxial gem.
The very big advantages of the polariscope coupled with the conoscope are that it has no refractive index limit like the refractometer and that it is also used on raw crystals. Interference figures are often quite hard to find in heavily colored gems.
To go further
the gemologist’s loupe
lighting
the tweezers
the darkfield loupe
the dichroscope
the spectroscope
the Chelsea filter and other filters
the refractometer
the microscope
the UV lamp
the karat scale