he dichroscope is a pocket-sized
tool that can aid in the identification of gemstones. This tool is used to
inspect pleochroism (leochroism is the change of color in colored anisotropic double refractive gemstones when viewed from different directions) in gemstones and to quickly separate some
common stones from each other (such as ruby versus red garnet).
Basic
There are two types of dichroscopes
on the market:
- calcite dichroscopes
- polarizing dichroscopes
In anisotropic gemstones,
different colors are absorbed in different directions ("directional
selective absorption") which causes the pleochroism that is observed with
the dichroscope.
Calcite dichroscope
The calcite dichroscope is the preferred type of dichroscope used in gemology. It works because the calcite rhomb (Iceland spar) separates the polarized slow and fast ray emerging from the gemstone. If you look at the viewing end of the dichroscope, two small windows are seen.
A gemstone is placed in front of the aperture (slightly touching it) with a strong white light source (such as a penlight) directly behind the stone. Light enters through the aperture of the dichroscope. The pleochroism colors inside the gemstone are separated by the calcite rhomb. The glass prisms on either side of the rhomb are there to guide the light straight through the instrument.
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Calcite dichroscope
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| Calcite Dichroscope |
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| Calcite Dichroscope |
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| view in calcite dichrosoope |
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Polarization dichroscope
Two polarizers
Pleochroism may also be observed
with the use of a polarizing filter. The disadvantage is that one will
only see one of the pleochroic colors at a time, making subtle changes of
colors (shades) difficult to recognize. This can be overcome by placing two
polarizing filters close together, each orientated 90° to the other (one in
North-South and the other in East-West position).
Although this kind of dichroscope
is very economical, the results obtained by them are less clear than in the
calcite ones. Stones with weak pleochroism will be hard to determine with this
type of tool.
The London dichroscope is a popular brand.
pleochroism in polarization dichroscope |
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| pleochroic stone in london dichroscope |
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Proper use and possible observations
Inspection
directions
- through the table
- through the crown facets
- another direction through the crown facets
- through the girdle
- a different direction through the girdle
There are three possibilities to
test the gemstone in Dichroscope
1. the colors in both windows
remain the same in all directions
the stone is isotropic (single refractive)
2. in total 2 colors are observed
the stone is anisotropic uniaxial
3. in total 3 colors are seen
the stone is anisotropic biaxial
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| Pleochroism in different gemstones |
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