The dichroscope is a very practical field tool that fits in a pocket. We have already seen with the polariscope that a white light ray passing through an anisotropic gem breaks down into two distinct rays, vibrating at 90 ° to each other. The dichroscope is used to separate the color of each light ray that passes through an anisotropic gem. In fact, when we look at the light transmitted through an anisotropic gem (outside its optical axis), we observe a single color, which is actually made up of two colors. The human eye cannot distinguish, so a dichroscope is used to separate these two colors. Here is for example the reaction of a synthetic ruby to the London dichroscope:
We can clearly see here the separation of the two colors: bright red and red-mauve. It is quite complex to retain the reactions of stones with a dichroscope because they are numerous, depend on the stone and also on its color saturation. However, this makes it easy to distinguish stones from a batch (synthetic spinels in a batch of sapphires for example). The reactions are sometimes quite weak and hard to observe, especially with light stones such as aquamarine. On the other hand, the fact of seeing two colors necessarily impliesan anisotropic gem, which makes it possible to distinguish a spinel from a sapphire for example. The spinel being isotropic, whatever the direction in which it is observed, it will present only one color while the sapphire will present two. Another case, we will observe three colors in a tanzanite, which will prevent it from being confused with another mineral species.
There are two families of dichroscope: the calcite dichroscope, which looks like a tube, and the London dichroscope, which looks like a filter.
First the London dichroscope. It is composed of two polarizing filters stuck one next to the other, but with the stripes at 90 °. This allows the colors to be filtered by allowing only those vibrating in one direction to pass (see photo above) It is very practical for stones of fairly large size and up to a certain distance (5 to 10 cm)
The calcite dichroscope is a tube in which a calcite crystal separates the incoming light ray into two separate light rays. We observe two windows (see photo below. When we observe an anisotropic gem through the calcite dichroscope, a different color appears in each window.
There are two important things to remember when using either of these dichroscopes. The first thing is that we always look at the transmitted light, that is to say the light which has passed through the stone. The corollary of this is that the dichroscope is perfectly useless for opaque stones. The second important thing is that if we observe the stone along its optical axis, we will only see one color. It is therefore important to rotate the dichroscope on itself, as well as the stone, to ensure that we are indeed facing an anisotropic gem. These precautions taken, the use of this instrument is childish and will not pose any problem.
To go further
gemologist’s loupe
lighting
tweezers
darkfield loupe
polariscope
spectroscope
Chelsea filter and other filters
refractometer
microscope
UV lamp
Carat scale