What Is A Dichroscope And How Is It Used?


Calcite Dichroscope
Calcite Dichroscope

A pleochroic gemstone displays different colors when viewed along different directions of the crystal. A dichroscope is a handheld tool for viewing the different colors or shades of a pleochroic piece of rough. This makes it an essential part of any gemologist’s or gem cutter’s toolkit. It has three main uses:

  • Helping with gemstone identification. Determining whether or not a specimen is pleochroic can help make some quick separations.
  • Showing the true colors of a pleochroic gem along each axis. A gem cutter can see the different colors and then visualize what they will look like on the finished stone.
  • Orienting a gemstone before cutting. A gem cutter can identify the gemstone’s strongest color and cut accordingly for the best result.

Where Can You Get A Dichroscope?

Dichroscopes are inexpensive tools readily available online. There are two types: those that show a split image of the gem you’re viewing though polarizing filters, and those that show the image through a piece of optical grade calcite. Either type works well when viewing gemstones. (Individuals inclined to DIY can even make their own instruments).

Holding the gemstone rough up to the dichroscope
Holding the gemstone rough up to the dichroscope

Using a Dichroscope To Identify Gemstones

Pleochroic gems can be dichroic (showing two colors) or trichroic (showing three colors), depending in part on their crystal systems. Non-pleochroic gems, of course, show only one color. Some rough gemstones look alike despite these differences. A quick dichroscope examination may be able to distinguish these.

For example, tourmaline and garnet rough may overlap in color. However, tourmalines are dichroic. Garnets are not. (Color change garnet can show an anomaly sometimes but are rare). So, you can tell the difference between a piece of tourmaline and garnet by looking for the one that has dichroism.

Dichroscope and rough
Dichroscope and rough

Typically, a garnet will just look red in both windows when viewed through a dichroscope. Tourmaline will look red and purple (or pink, orange, etc). In the picture below, you can see the two windows of calcite inside the scope. Notice the color difference. One is hot pink. The other is a darker purple/pink. This stone is likely not a garnet.

View of the rough through the dichroscope
View of the rough through the dichroscope

Trichroic stones like andalusite will show all three colors, but only two at a time while you rotate the rough.

It’s a good idea to carry a dichroscope with you at gem shows. A piece of garnet is a lot cheaper than a piece of ruby, and it’s not uncommon for them to be mixed. They are often found in the same areas and mines. (Identifying the stones in rough parcels can be difficult). Dichroscopes can’t eliminate all possibilities, but they can help you make some quick eliminations when you can’t perform complete examinations.

Using A Dichroscope To Visualize Gemstone Colors Before Cutting

In my experience, faceters have tended to overlook the dichroscope. However, it’s an ideal tool for getting an idea of how the colors will look on a finished pleochroic gem. For example, have you ever been unsure how to cut a piece of tourmaline rough? A dark or different colored “c” axis? The dichroscope can help you decide. It’s a great learning tool for inexperienced gem cutters.

In the picture below, notice that this green tourmaline has a blue/green axis and an olive green axis. This stone can be cut into a nice, bright, green gem, if faceted with the blue/green axis up on the table. In other words, if you mix the blue/green and the olive green, you’ll get a good idea of what the finished stone will look like.

View of a piece of green tourmaline through a dichroscope
View of a piece of green tourmaline through a dichroscope

Hopefully, the rough will allow this cut. Keep in mind that other factors, like design shape and type, will influence the result. Nevertheless, in general this will give you a good idea of how the axis colors will mix. Look at them and imagine what color they would combine to create.

Using A Dichroscope To Orient A Gemstone

By looking through the scope and turning a piece of rough, you can identify the axis that shows the strongest color (or the color you prefer) and orient the stone to achieve it. For example, if the colors of a tourmaline were shown to be olive green and brown, you could be pretty sure that the color of the finished stone would be a muddy green. You would need to consider cutting a design that would minimize the poor color “c” axis, like my “Smithsonian Bar,” which has very steep ends to help keep the “c” axis from mixing and darkening the finished gem.

A dichroscope is very helpful when orienting sapphire gemstones. The light should be coming from over your shoulder in these cases. When you see the best color up, that is the direction that you will generally want to show through the table of the finished stone.

Additional Information On The Dichroscope

For more information on how to use the dichroscope and interpret your test results, see our Premium article on this instrument. The International Gem Society (IGS) has many articles on gemstone identification techniques, tools, and testing strategies. For more information on evaluating colors, consult our article on gemstone color wheels.

Common Gemstones And Dichroism

“Andalusite” by Moha112100 is licensed under CC By-SA 3.0
“Andalusite37” by Moha112100 is licensed under CC By-SA 3.0
Gram Faceting Archive of Information
This edited version of an article by the late Jeff Graham is part of a special archived informational series from Gram Faceting. Used with permission.