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The word "DICHROSCOPE" really means "to see two colors". The use of a dichroscope can be helpful in identifying colored gemstones. Also, faceters of gemstones may find one useful for favorable orientation of rough crystals.

Gemstones which are single crystals transmit polarized light which can have different colors (usually shades of one color) depending on the direction of the light ray through the crystal. It is the crystal structure and the presence of usually metal ions (e.g., Fe, Cr, V, Mn etc) which absorb certain parts of the visible light spectrum and allow the transmission of the residual light thus producing a distinctive color for the gemstone.

For a gemstone to exhibit dichroism it must be a single crystal (not polycrsytalline) and not of cubic or isometric crystal symmetry (this excludes diamond, spinel, cubic zirconia, garnet etc) . If a crystal has isometric symmetry then there is no possibility of light being absorbed differently no matter what direction it takes. Ditto glass imitations and amorphous materials (e.g., opals and metamict zircon).

A dichroscope is little (Approx 2 inch) optical gadget through which you view an illuminated colored gemstone from various angles in the hope of detecting two colors. If you do detect two colors then with your experience of gem testing (or reference to a gem book) you may narrow down the possibilities for identification.

A dichroscope have a little calcite rhomb inside it which provides a view of the two colors side-by-side, or you can make one yourself of similar design. Also you can use a piece of polaroid (cut from sun glasses as default ) but then you have to remember the color from one turn to the next.

Calcite crystals are unique (almost) in that they have a very high double refraction of 0.172, which is the maximum difference in refractive indices, depending on the direction the light rays take through the crystal; this is high compared to the D.R. of 0.009 for quartz.

As a gemologist you have to know which gemstones exhibit appreciable dichroism.  Top of the list is tourmaline, which when deeply colored, displays very strong dichroism. Tourmaline has trigonal symmetry and shows the deepest color when viewed down the prism or optic axis (O-ray), and the lightest color when viewed across the prism (combined O & E-ray). Use a piece of polaroid to check the colors across the prism whereby turning the crystal you can see the different colors of the 0-ray and E-ray. Using a dichroscope you see via the pinhole, both colors side-by-side. Experiment with tourmaline using natural prism specimens and then with faceted stones until you are confident about what you are observing.

It is helpful to have a little collection of faceted gemstones exhibiting dichroism of varying degree, for comparison with unknowns and testing purposes. This would include synthetic ruby, sapphire and emerald, tourmalines, various beryls and quartzes.

Letís say you examine a faceted red gemstone and you detect dichroism, a deep red and pink colors. This means it is not glass, spinel or garnet, but could be ruby or tourmaline. Further tests are required for identification. If you don't detect dichroism then you canít really say anything for sure. You may be looking down an optic axis of what is usually a dichroic mineral, or it may not be present (e.g., citrines are not always dichroic).

Gemstones that often exhibit strong dichroism include tourmaline, tanzanite, andalusite, apatite, benitoite, ruby, many sapphires, alexandrite, blue zircon, epidote, iolite, kyanite, sphene and many more rarities. Some of these gemstones may vary in color (hue) depending on the type of light illuminating them (e.g., daylight, tungsten filament, fluorescent).

Gemstones that exhibit noticeable (distinct) dichroism include amethyst, citrine, emerald, aquamarine, chrysoberyl, and topaz.

You can only see two colors at once. However, some colored crystals of low symmetry (e.g. orthorhombic, monoclinic, triclinic) are called pleochroic in the sense that they can exhibit three colors depending on the direction of light through the crystal. A good example of this is andalusite (a rare gemstone) and the colors observed are green, brown and red. The valuable gemstone, alexandrite, is another example, the colors being deep red, orange-yellow and green.

So a dichroscope is quite useful in identifying a gemstone. Always have one available. By using a hand lens and a dichroscope you should be able to identify perhaps 90% of all gemstones that come to hand. To do better than that you would need more costly testing equipment (e.g., binocular microscope and refractometer). However, much can be achieve by using simple equipment. This is what makes gemology a challenge and a fascinating hobby.
Dichroscope 7
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