We are going to look at the gemmologists and jewellers dichroscope used to test coloured
gemstones.
They are very simple to use, it's easy with just a little practice to see and interpret
a result, and basically you have only 3 possible outcomes from the test.
It will allow you to identify if a gem is 1.non-pleochroic, 2.dichroic or 3.trichroic
It is used to detect and if present view pleochroism in gemstones.
Pleochroism if present in a gemstone is colour or subtle shade of colour variations when
the gemstone is viewed in different directions using the same light source.
There are a few materials where it is easily visible with the naked eye, you rotate the
gem and it changes colour or shade of colour, but with most materials the unaided human
eye cannot easily detect it.
So we use the dichroscope to test gemstones for it.
By itself it is a simple test that can be used to distinguish one gemstone from another,
and when used in conjunction with a series of other simple tests will aid in the identification
of a gemstone.
There are two types of dichroscope available Calcite dichroscopes like this one
Polarizing dichroscopes like this one
Using either of them is simple -- you simply take a gemstone put a good white light source
behind it and then look at the gemstone through the dichroscope.
With the calcite type you hold it right up as close as you can to the gemstone.
With the polarizing type you can see a visible line across the centre of the filters, you
need to keep that line in the centre of the gem and any colour differences will be dissected
on either side of that line.
Then you rotate the dichroscope slowly through at least 180 degrees observing the colour
of the gemstone. 1 of three things will happen
1.
The colour will remain the same -- meaning its non-pleochoric
2.
You will see a maximum of 2 colour variations which means the gems dichroic
3.
You will see a maximum 3 different colours, that's 3 in total, you will only see 2 at
a time but one will change at different angles giving 3 colours in total which means its
trichroic If present the pleochroism or colour difference
can vary from distinct to weak not only in different materials but also in different
specimens of the same material so examples are just a guide.
You need to do the test holding the gemstone at different angles and repeat the rotation
of it observing the colour.
You need to check the gemstone at different angles because gems that display 2 colours
will held in at 1 certain angle only show 1 colour and the gems that can display 3 colours
will only display 2 at a time and the angle needs to be changed to see the 3rd.
So by changing the angle of rotation you're ensuring you have seen the maximum amount
of colour variations possible and not accidentally the least amount.
So you need to do the test holding the gemstone at different angles and repeat the rotation
of it observing the colour.
By different angles I mean if it's a cut gem look at it through the table and rotate, then
look through the crown facets and rotate and look then through the girdle and rotate.
Ifs it's a crystal or cabochon then you will need to imagine where those angles would be
so you can observe it in the different directions.
So a dichroscope alone will not conclusively tell you what a gemstone stone is, but it
will tell you identifying distinguishing properties.
So it can be used to simply and rapidly to separate similar looking coloured stones and
is used in conjunction with other portable equipment in a series of tests to support
identification.