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


This part will focus on the calcite type dichroscope.

As with any other instrument, proper use will determine if you get accurate results.
The stone is held close to the aperture, even slightly touching it, with a strong allochromatic, white light source behind the stone. When one looks through the other end of the dichroscope two small windows next to eachother will be seen with a color in each of the windows. These colors may either be the same or different colors or different shades of colors.
Now turn the dichroscope slowly 180° between your fingers whilst keeping the stone in a fixed position and observe if the colors in the two windows are the same or will differ.


This procedure should be repeated another 4 times from different angles.

  1. through the table
  2. through the crown facets
  3. another direction through the crown facets
  4. through the girdle
  5. a different direction through the girdle

Of course for en-cabochon cut stones there are no tables and crown facets, but the inspection directions will be the same.

After making a mental image of the colors seen, there are three possibilities.

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

In the images below (for isotropic, uniaxial and biaxial) two out of five observations are given.

Isotropic stones

Dichroscope5.png


When the colors in both windows remain the same after 5 readings (2 of them are illustrated), the stone will be single refractive. In this image the colors are red but they could be any color.
Not many colored stones are single refractive, so this will narrow down them quite a bit. There are however exceptions to the rule as heated Citrine.


Uniaxial stones

Dichroscope4.png


Here 2 different colors are seen (red and orange) after 5 readings (two readings illustrated). This means the stone is uniaxial.
Again some exceptions to the rule excist. Low type Zircon may show up as isotrope, whilst Peridot (which is biaxial) may appear uniaxial.


Biaxial stones

Dichroscope3.png


When in total 3 colors (or shades of color) are seen after 5 readings, the stone will be biaxial without exception.
Here the colors are red, orange and yellowish-green but they can be any colors or shades of the same color. Careful observations may even enable you to distuinguish between natural Tanzanite and heated Tanzanite (Zoisite).

When three colors are seen, we name it "trichroic" (tri = 3, cho = color).


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