Quite simply: a tool used to see if your gemstone is singly or doubly refractive.
The dichroscope is a tool that helps gem collectors in discovering if the gem they are looking at is a singly refractive gem or a doubly refractive gem. When light enters a gemstone, depending on its properties the light will either travel as a single ray of light within the gem (singly refractive gems) or will divide in two (doubly refractive gems). Calcite for example massively splits light as it enters the mineral and it is in fact Calcite that is used as the lens in popular dichroscopes to aid the detection of double refraction (also known as birefringence).
But why do we need to know if a gem is splitting light in two? Well, it is one of the best ways of identifying certain similar-looking gems. For example Garnet is always a singly refractive gem, whereas Ruby is doubly refractive. As only a few gemstones such as Diamond, Garnet and Spinel, along with many synthetics such as CZ and glass are singly refractive, the dichroscope is very useful indeed for spotting fakes! Probably the most well-known doubly refractive gemstone is Zircon.
It can also be helpful for lapidarists, as they can use the dichroscope to analyse a piece of rough and spot which colours are produced from a doubly refractive gemstone, so that they know where to cut the gemstone to showcase the different colours.
As light splits in two inside doubly refractive gems, different colours are produced. This occurs as the split rays of light will travel at different speeds and at different angles; depending on the level of these variances it will affect the colours that are produced. Even with the naked eye you can often see this effect. If when rocking a gemstone back and forth you see two different colours, the gemstone is referred to as dichroic; if you can see three different colours then it is known as a trichroic gem.
However, to complicate life a little, don’t confuse different colours seen in a gem due to diochroism with the dispersion of light. For example, as a Diamond is a singly refractive gem, the different colours coming out of a quality white Diamond are due to dispersion and not diochroism. However, as it’s not always possible to see with the naked eye the effects of diochroism: this is where this very important gem tool comes to our aid.
When you master the use of the dichroscope, if the gem is doubly refractive, when you look into the eye piece, as you rotate the dichroscope through 360 degrees, at one point or another you will see two distinctly different colours (or two very different shades of the same colour) in the two rectangular boxes at the bottom of the tube.
If you don’t find two different colours or shades then reposition your gem in the tweezers and look at it from a different angle. Try it with the table of the gem touching the dichroscope first. If you don’t see the different colours, put the crown facet next to the dichroscope and start rotating it again. If you still don’t see what you are looking for then place the pavilion facets next to your tool and begin rotating once again.
Look into your dichroscope as you would your loupe, making sure to look through the right end, you will know you have done this as you will clearly see two rectangle shapes.
It’s incredibly simple to use once you have mastered holding your torch, dichroscope and gemstone with tweezers! If you struggle with this, try and clamp your torch so that you can hold your tweezers in one hand and your dichroscope in the other. Alternatively if you have a strong light from a ceiling light this will often also light up the gem sufficiently to see the two different colours.
