Dichroscope
Gemstone Testing Tools
By:Ritika Changrani
Summary: Dichroscope is a
tool used to differentiate similar
looking gemstones based on the
Dichroic property of the gemstones.
Some gemstones are singly
refractive. They exhibit only one
colour to observer. There are also
gemstones which exhibit two
different colours of light. But most
of that is not detected by the human
eye since the two colours produced
are so close in wavelength that it
becomes difficult for the human eye
to identify them. The two colours in
such gemstones mix up to give a
slightly different colour. Such
stones are called doubly refractive
gemstones. For example, ruby is made
up of dark red and light pink
colour. But it appears to the viewer
as light red.
 The
property of the gemstone to exhibit
two different colours or shades when
turned in two different directions
under light is called as dichroism.
Examples of the gemstones which are
Dichroic are
sapphire,
zircon,
tourmaline and
topaz.
Dichroscope is a tool used to
differentiate similar looking
gemstones based on the Dichroic
property of the gemstones. Hence it
can easily differentiate gemstones
like ruby from tourmaline since
ruby
is Dichroic and tourmaline is
pleochroic. Dichroscope also helps
to determine Pleochroism and
trichorism. Dichroscope helps to
determine the optical properties of
a gemstone. (image courtesy:
www.faceters.com)
Dichroscope is a metal tube which
is open on one end and has a lens at
the other. It has an optical calcite
mounted inside the tube and produces
a double image of the square
opening. Enough bright light is
directed on a gemstone and viewed
through the dichroscope. Dichroscope
separates the different colours of
light into its constituent colours.
Light entering the dichroscope is
broken into two polarized rays. Two
images of different colours indicate
that the vibrations are of different
wavelengths and at right angles to
each other. If dichroism is not
detected in the first test, the
gemstone has to be turned and output
colours must be viewed in other
directions too.
Pleochroism can be detected by
rotating the instrument at an angle
of 90 degrees. If the two colours
switch sides on the split image, the
gemstone is pleochroic. To determine
trichorism, it is necessary to
change the orientation of the stone.
If one new colour is determined
along with one of the colours of
previous orientation, the gemstone
exhibits trichorism. Trichroic gems
show three different colours when
viewed in three directions
perpendicular to each other. Singly
refractive stones produce only one
colour on the dichroscope.
How does a Dichroscope work?
Dichroscope works on the principle
of plane polarized light. Plane
polarized light is the light of a
specific wavelength that is
separated from ordinary light and
travels in a specific direction.
Some waves may travel up and down
and some side to side. When light
traveling in different direction is
being separated, it is possible to
view the individual colours of
specific wavelength.
Types of Dichroscopes
There are two different types of
dichroscopes. They are London
dichroscope and Calcite dichroscope.
London dichroscope
In a London dichroscope, two pieces
of Polaroid orientated at right
angles to each other are placed such
that the light entering each
polarizing filter are at right
angles to each other. The two
polaroids in a London dichroscope
are joined together in same plane.
London dichroscopes are much cheaper
when compared to the calcite
dichroscope but does not give as
clear a result as the other. It is
difficult to determine gemstones
with weak Pleochroism with a London
dichroscope.
Calcite Dichroscope
The calcite dichroscope is made up
of a calcite crystal. The calcite
has a special characteristic called
bi-refringent. This property of
calcite enables the gemstone to
appear doubled when looking through
it. The calcite crystal splits the
light entering the dichroscope into
two rays and thereby helps identify
the difference in colours. The two
rays form two images at right angles
to each other.
Out of the two dichroscopes, the
calcite dichroscope is the most
commonly used in the gem testing
laboratories to test the Dichroic
property of the gem stones.
How to use a Dichroscope?
Hold the upper portion of the
dichroscope near the eye - may be an
inch from your eye. Place the
gemstone near the opening. The other
end of the dichroscope is hexagonal
and hence it is easier to turn the
dichroscope while testing. View the
gemstone through the gem view
opening. Slowly rotate the
dichroscope to complete one full
rotation and observe the colour
difference of the two small squares
which appear inside the dichroscope
while you rotate. If you observe two
colours while you rotate, the
gemstone is dichroic. If three
colours are observed, the stone is
trichroic. Repeat the observation to
confirm the number of colours
observed. Observe the strength of
the colours as it helps determine
the type of gemstone. Always make
sure that sufficient bright light is
focused on the gemstone while you
observe the colours using
dichroscope. Never use fluorescent
light since it can give a false
result.
Advantages
* Dichroscope helps determine the
pleochroic property of the gemstone
which helps to separate different
types of gemstones.
* It also helps to determine whether
the gemstone exhibits weak
Pleochroism or strong
Pleochroism.
* Dichroscope also helps identify
some gemstones.
* It also helps to study the optical
properties of the gemstone.
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