Dichroscope

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The dichroscope is a pocket size tool which can aid in the identification of gemstones. With plenty of experience one could even positively identify some gemstones with the dichroscope alone (although confirmation tests should always be carried out).
This tool is to inspect pleochroism and associated colors in gemstones and to quickly seperate some common stones from eachother (as Ruby versus red Garnet).

There are two types of dichroscopes on the market

  • Calcite dichroscopes
  • Polarizing dichroscopes

In anistrope gemstones different colors are absorped in different directions ("directional selective absorption") which causes the pleochroism that is observed with the dichroscope.

Calcite dichroscope

Calcite dichroscope

The calcite dichroscope is the prefered type of dichroscope used in gemmology. It works because the calcite rhomb seperates the polarized slow and fast ray emerging from the gemstone.
A gemstone is placed infront of the aperture (slightly touching it) with a strong white lightsource (as a penlight) directly behind the stone. When one views through the other side of the dichroscope, two small windows are seen.

Light enters through the aperture of the dichroscope and the pleochrism colors inside the gemstone will be seperated by the calcite rhomb (made of Iceland feldspar). The glass prisms on either side of the rhomb are there to guide the light straight through the instrument.


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 one 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 clearer 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.

Proper use and possible observations

Inspection directions

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Sources

  • Gem Identification Made easy 3rd edition (2006) - A.C. Bonanno, Antoinette Matlins
  • Gemmology 3rd edition (2006) - Peter Read
  • Guide to Affordable Gemology - Dr. W. Wm. Hanneman
  • Ruby & Sapphire (1997) - Richard W. Hughes