Polariscope @ Jewel Info 4 U
By: Ritika Changrani
Summary: Polariscope is an
instrument used in gemology which
helps to find if the gemstone is
single refractive or double
refractive and also allows
determining the various crystal axis
of the stone.
A Polariscope is used to determine
whether the gemstone is natural or
synthetic.
This is one of the most commonly
used instruments by the
gemmologists. The use of plane
polarized light allows us to see the
actual path the beams are taking
through the stone.
 A polariscope is made of two
Polaroid plates that rotate, a power
switch, a stone holder and a bottom
light source. Two polarized filters
or plates are made of polarizing
plastic sheets, one is on the top of
the instrument, known as analyzer
and the other is on the bottom of
the instrument, known as polarizer.
Polarizer and analyzer have their
own vibrational planes. When the
vibrational plane of polarizer and
the vibrational direction of the
analyzer are at right angles to each
other, the field between them
appears dark. This position is known
as crossed position and it is in
this position that the gemstones are
tested to see if they are isotropic,
anisotropic, anomalously double
refractive and anisotropic
aggregate. The polariscope can be
used to determine the optical
character as well as the optical
sign of the gemstone.
Polariscope can also be used to
determine the strains in diamond. It
helps in separating natural
gemstones from synthetic ones and
also helps to distinguish solid
inclusions from negative inclusions.
Polariscope can also be used for
recognizing polysynthetic twinning.
How to use a Polariscope?
- The polariscope is adjusted for
cross position by setting the
vibrational plane of polarizer and
the vibrational direction of
analyzer at right angles to each
other.
- Turn on the light source and place
the gemstone on the rotating
platform above the polarizer. The
gemstone is rotated in a vertical
axis.
-
Observe the gemstone through the
analyzer and note down the
observations
- Based on the observations the
nature of the gemstone can be
determined as follows.
- If the stone remains dark
throughout the 360 degree rotation,
the stone is said to be isotropic or
singly refractive.
- If the stone darkens evenly at
every 90 degree interval, the stone
is said to be doubly refractive or
anisotropic.
- If the stone appears light
throughout the rotation, it is said
to be microcrystalline or crypto
crystalline aggregate.
- If the stone shows anomalous
double refraction, the stone is
singly refractive. Anomalous double
refraction may be confusing at
times. Hence when there is a
suspicion that about anomalous
double refraction, it is recommended
to orientate the stone in the
position where it appears lightest.
Then turn the analyzer to 90 degree.
If the stone appears lighter than
before it is said to exhibit
anomalous double refraction and
hence is singly refractive. If the
stone appears the same, it is doubly
refractive.
How does a Polariscope work?Polariscope works on the principle
of plane polarized light. Single
refractive gemstones do not break
the plane polarized light into
various colours. There is only one
beam of light coming out. Examples
of singly refractive gemstones
include diamonds. Double refractive
gemstones break the plane polarized
light into two paths. One is the
ordinary beam and the other is the
extraordinary beam. Based on
analyzing the transmitted light, the
nature of gemstone can be
identified. Some of the doubly
refractive gemstones are quartz,
tourmaline, ruby, zircon, peridot,
sapphire and rulite.
Both the filters i.e. polarizer and
the analyzer are turned to the dark
position. When plane polarized light
is passed through the gemstone, the
singly refractive stone remains dark
since it does not change the path of
light. Hence it remains dark when
observed in all positions. But in
case of double refractive gemstone,
when plane polarized light is passed
through it, the stone changes the
path of the light and hence the
direction of the light wave changes.
This produces light that is no more
polarized. The change in direction
of light makes the stone change from
light to dark as it is rotated
between the Polaroid plates.
Since the polariscope works through
plane polarized light, it can be
used to test transparent as well as
translucent gemstones, but cannot be
used to test opaque materials. Once
the stone is found to be doubly
refractive, a conoscope can help to
determine the optic interference
figure. Optic interference figure
helps to locate the various
directions in which the light is
traveling through the stone. This
information in turn helps to
identify the type of gemstone. The
different shadow patterns observed
through the polariscope helps to
determine the crystal structure and
diagnostic patterns of the gemstone.
Types of Polariscope
There are two types of polariscope
i.e. plane polariscope and circular
polariscope. Circular polariscope
work on circular polarized light
unlike plane polariscope which works
on plane polarized light. In a
circular polariscope, two quarter
wave plates are added to plane
polariscope. One quarter wave plate
is placed between the polarizer and
the gemstone to be tested and the
second quarter wave plate is placed
between the analyzer and the
gemstone. The quarter wave plates
produce circularly polarized light.
The advantage of using circular
polariscope is that it helps
distinguish isochromatics and
isoclinics. But plane polariscope
are more extensively used for
testing gemstones.
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