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The Iceland spar dichroscope is a conventional instrument used specifically to observe the pleochroism of gemstones.

In some cases, pleochroism is also the basis for determining the variety of gemstones. Especially when instruments such as refractometers and polariscope cannot determine whether the colored gemstone is a homogeneous or inhomogeneous gemstone, the dichroscope can very effectively determine the optical characteristics of the colored gemstone.

Principle and structure

When natural light enters an inhomogeneous gemstone, it is decomposed into two polarized lights with mutually perpendicular vibration directions, and the propagation directions of these two lights are also different.The anisotropy of inhomogeneous gemstones causes the gemstones to absorb light with different vibration directions differently. As long as these two vibrations of light can be separated, different colors can be seen.

The most commonly used dichroscope is the Iceland spar dichroscope, which consists of a glass prism, an Iceland spar rhombus, a lens, a light-transmitting window, and an eyepiece (Figure 1). Iceland spar can distinguish the two plane-polarized lights passing through the gemstone and compare the colors of the two beams side by side.

Homogeneous gemstones do not have anisotropy, so there is no pleochroism, and the colors observed in the two windows are the same. Inhomogeneous colored gemstones have anisotropy, so there is pleochroism.

Figure 1  Iceland spar dichroscope structure

Instructions
  1.Operating Steps
1）Use white light to shine through colored gemstones;

2）Place the sample in front of the objective lens, as close to the objective lens as possible,to prevent the surface reflected light of the sample from entering the small hole;
3）Keep your eyes close to the dichroscope and rotate it while observing, paying attention to the color changes in the two windows;
4）Rotate the sample and observe from all directions.
2.Phenomenon and Conclusion
1）If only one color always appears, and the sample is isotropic (homogeneous gemstone);
2）If two colors appear, the sample is a inhomogeneous gemstone;
3）Three colors appear, the sample is an inhomogeneous gemstone, and it is a biaxial crystal.
3.Pleochroism Grades and Descriptions
Describe from two aspects: intensity and color
Intensity is usually divided into four levels:

1）Strong: Pleochroism can be seen with the naked eye, such as cordierite (Figure 2), andalusite, etc.

Figure 2 Pleochroism of cordierite
2）Obvious: It is difficult to observe with the naked eye, but the pleochroism is obvious in a dichroscope, such as ruby (Figure 3);

Figure 3 Ruby pleochroism

3）Weak: Pleochroism can be seen in the dichroscope, but it is not obvious, such as amethyst and olivine (Figure 4);

Figure 4: Pleochroism of olivine
4）None: Pleochroism cannot be observed under a dichroscope, such as spinel, garnet (Figure 5), etc.

Figure 5: Pleochroism of garnet

   Usage
 1.Helps identify gemstones with strong pleochroism: such as cordierite with significant trichromaticity (blue, purple-blue, light yellow);
2.Distinguishing between isotropic and anisotropic gemstones: For example, ruby and red spinel are very similar in appearance, but the two can be effectively distinguished by observing pleochroism. Ruby has obvious dichroism, while spinel is isotropic and has no pleochroism.
 3.Guiding processing: Understanding the pleochroism of gemstones is very helpful for the orientation of gemstone cutting. For example, the top facet of ruby should be perpendicular to the optical axis during processing so that the best color of the gemstone can be displayed through the top facet.
Precautions
（1）The sample must be a transparent, colored gemstone with birefringence;
（2）The light source should be ordinary white light or sunlight for adaptive illumination, and monochromatic light or polarized light should not be used;
（3）The gem should be placed as close as possible to one end of the dichroscope, and the eye to the other end;
（4）When the dichroscope is rotated to observe at a certain position of the sample, the colors of the two windows are interchanged, which is the manifestation of polychromaticity. If the colors of the two windows are different, but there is no exchange phenomenon when the dichroscope is rotated, it may be caused by the entry of additional polarized light.
（5）Color banding of the sample, uneven color zoning, not to be confused with pleochroism, such as uneven color in sapphire, uneven color in tourmaline;
（6）When observing, turn the gem or dichroscope and observe from all directions;
（7）If it is a trichromatic gemstone, its three colors are displayed in different directions. When observed from one direction, only two colors can be seen.
（8）The strength of gemstone pleochroism has nothing to do with the size of birefringence, such as: olivine (weak), cordierite (strong);
（9）In gem identification, pleochroism is only used as auxiliary evidence and must be supported by other testing methods.  

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