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Making a polariscope from sunglasses


#1

Was: South American Topaz?

Hi Peter

Could you expand on making a polariscope from sunglasses…and what
one would be looking for re…stone identification.

Tkx
Simone


#2
Could you expand on making a polariscope from sunglasses....and
what one would be looking for re..stone identification. 

A polariscope is simply two pieces of polarizing material oriented
one over the other, with a gap in between for the test sample, and
the two pieces oriented at right angles to each other. Some sort of
light source, perhaps a flashlight, is set underneath the whole
thing. Importantly, the polarizing pieces have to be a linear
orientation, rather than circular (such as some camera filters use).
In this orientation, looking through the two lenses, most light will
be obscured, since light that can pass through the first lens is all
mostly polarized in one direction, and the next lens, orented at
right angles, then blocks it. If you put a piece of transparent
material in between, this changes if that material itself has an
effect on the polarization of light. Many materials do. These
materials don’t simply block light that’s not oriented to their
structure, but “twist it”, effectively altering the polarization
direction of the light passing through. There’s a limit to how much
this can be done, so light with a polarization direction at right
angles to the material will be blocked, while light with a
polariaztion direction at some lesser angle, will be admited to an
increasing degree as the difference in polarization direction
decreases. The polaroid lenses, by the way, are doing exactly the
same.

Anyway, if you have a piece of material which is singly refractive,
such as glass, or most other non-crystaline materials, or those which
crystalize in the cubic system (diamond, garnets, spinel, to name a
few), these materials don’t polarize light, except for polarization
effects seen from internal strain. Placed between the two test
lenses, they appear dark, except perhaps for patchy uneven areas of
some brightening caused by that strain. No major or dramatic change
as you rotate the material.

By contrast, doubly refractive materials or minerals all have more
than one polarization direction, and light entering them is split
into two componants, each with it’s own polarization direction, and
each responding to an often slightly different refractive index. To
do this, each portion of the light beam has been twisted to conform
to that particular polarization direction. When you place these
materials between the polariscope lenses and rotate them along the
axis of the whole assembly, when the polarization directions of the
test lenses and the material being viewed line up, light again does
not pass, and the sample appears dark. At other rotational positions,
though, because the material has twisted the light that gets through
the first lens, it then is no longer at a right angle to the second
lens, and some portion gets through, so the sample then appears
light. Essentially, as you rotate your sample, the stone appears to
blink on and off, light to dark to light again. You need to test at
more than one rotational direction, since in some materials (uniaxial
systems, like the hexagonal system), there is one optic axis /
rotational direction where no blinking is seen.

Essentially, what this instrument can tell you is some of what
crystal class the material belongs to, ie cubic or other
non-polarizing material, versus those materials that are in other
crystal classes. With experience and a bit more selective
observation, you can also distinguish between uniaxial and biaxial
minerals, further narrowing an identification. Also, the polariscope
can be useful for observing pleochric colors.

In this particular case, the useful application would be that a
polariscope is a very fast way to check to see if an unknown material
is glass rather than topaz or quartz, the two most obvious
possibilities for actual gem material, since both of them are double
refractive, and “blink” nicely under the polariscope.

This is only a very rudimentary discussion of polariscopes and what
you can determine with them. Do some more online searching, and you
can find more, or any decent gemology text will discuss it in detail.

Aside from speed, one of the things that makes polariscopes so
useful is also that they are cheaper even to buy ready made, than
things like spectroscopes or refractometers (which are usually the
work horses of gem ID after a good loupe or microscope) And as I
said, if you need, you can rig up a makeshift one from most decent
polarizing sunglass lenses, or you can buy sheets of polarizing
material for not a lot of cash from places like Science Surplus and
Supply (I think that’s the name) or Edmunds scientific. Both have web
sites.

Peter


#3

Hi Simone,

Here at Ganoksin is a description of making your own polariscope and
mentions how to make the sunglass version.

best
Charles


#4

Making a polariscope from sunglasses isn’t difficult.

  1. Make sure the sunglasses have polarized lenses.

  2. The lens that would be the equivalent of the base lens on a
    polariscope needs to be fixed in a manner that a light can show from
    the side opposite you.

  3. Hold the second lens or arrange a means to hold it in a way that
    it can be rotated.

  4. Hold the gemstone between the lenses.

  5. Do the deed.

This is pretty basic and obviously has no method for magnification,
but is certainly open to further innovation and gadgeting.

Mike DeBurgh, GJG
Henderson, NV


#5

Hi Charles et al

Thank you. Now I know how to make it and it is simple, but what am I
looking for? I would presume that different gem stones appear
differently under this polarization.

Is there a chart or description of the stones under this view?

I always learn so much. Thank you.

Simone


#6
Thank you. Now I know how to make it and it is simple, but what am
I looking for? I would presume that different gem stones appear
differently under this polarization.
Is there a chart or description of the stones under this view? 

Any good gemological text will show you the different effects you can
see. So, for that matter, does that article Charles pointed you to
here on the Ganoksin web site. After that, you can simply refer to
any decent gem reference which will tell you which optic class any
given gem is in. For example, once you determine that a given green
stone is double refractive, and uniaxial, you’re ruled out a number
of things, and the possibility that you have an emerald instead of a
tsavorite or a glass, can be decided upon. It’s not that the emerald
looks unique for emerald. it does/looks roughly the same as any other
uniaxial gem, and there are a number of them, including green ones.
But by eliminating possibilities that are not in this crystal system,
you often have all the info you then need.

A polariscope is not an instrument that diffentiates all gems from
all others. It identifies the optic class of a gem material. There
are many gems that all do the same thing.

The biggest difference is in simply seperating singly refractive
from doubly refractive gems. it turns out that this is an important
distinction, since glass is singly refractive, and usually shows
characteristic strain patterns. So does synthetic spinel and plastic.
While there are many other identification procedures and distinctions
you may wish to make, the simple ability to quickly seperate out a
large number of the fakes (glass) makes this a very useful tool. Take
it further to more advanced methods, and you can get more specific.
But the polariscope, with a few exceptions, seldom tells you exactly
what a gem is. Rather, it is often capable of drastically narrowing
the range of possibilities, and often that is all you need to make
your determination by adding in other visual clues.

Peter


#7

The polariscope is simply a tool to help identify a characteristic
of a gemstone, or lack of that characteristic. That characteristic in
combination with others such as the refractive index, color,
opacity, inclusions, luster, fracture pattern, etc., etc., etc., aid
in identifying a gemstone. Even with all the data identification can
still be a difficult matter though.

Mike DeBurgh, GJG
Henderson, NV