The Dichroscope: A Faceter’s Guide

Editor's note: you can also learn how to use a dichroscope for gem identification. The International Gem Society (IGS) has many articles on identification techniques and tools.

Where Can You Get a Dichroscope?

You can easily find this tool for sale online. There are two types: those that show a split image of the gem you're viewing through polarizing filters, and those that show the image through a piece of optical-grade calcite. Either type works well when viewing gemstones. DIY-inclined individuals can even make their own instruments.

Making Quick Gem Separations with a Dichroscope

By itself, a dichroscopic test can't identify a gemstone, but it can help you quickly eliminate some possibilities. This is especially useful in situations when you can't perform a complete examination — like at a gem show. This tool is very easy to carry and can save you some headaches and money.

For example, it's not uncommon to find garnet rough mixed with ruby rough at gem shows. These stones can overlap in color (red) and often occur in the same areas and mines. Identifying stones in rough parcels can be difficult (and ruby is typically a lot more expensive than garnet). Tourmaline and garnet can also overlap in color (many different kinds) and may be easily confused as rough.

How can a dichroscope help you quickly distinguish these stones? Garnets are singly refractive, so they're not pleochroic. Rubies and tourmalines, on the other hand, are doubly refractive and can show pleochroic colors. When viewed through a dichroscope, garnets will most likely just show one color. Rubies and tourmalines can show two colors.

Is This a Garnet?

Take a look at the rough gem below. When viewed through a dichroscope, you'll see two windows, each showing a color. You can see that this piece shows hot pink in one window, but a darker purple/pink in the other.

This is most likely not a garnet. If it were, you'd typically see the same color in both windows.

With just this test, you can't positively identify the gem. However, you've eliminated at least one possibility.

Visualizing Pleochroic Gemstone Colors Before Cutting

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 dealing with pleochroic rough.

In the picture to the right, notice that this green tourmaline has a blue/green axis and an olive green axis when viewed through a dichroscope. 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.

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.

My articles on cutting gem rough with different colors on different axes and using a color wheel can also help you visualize color mixes.

Using a Dichroscope to Orient a Pleochroic 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 also 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's the direction that you'll generally want to show through the table of the finished stone.

Common Gemstones and Pleochroism

If the stone you're examining shows just one color through the dichroscope, it's most likely singly refractive. (Editor's note: you won't see pleochroic colors if you're looking down the optic axis of a gem, even if that gem is pleochroic. You must be sure to view a stone from many optical directions before you conclude it's most likely singly refractive. See our article on the dichroscope for gemologists for more information).

Depending in part on their crystal system, doubly refractive gems can show two colors (dichroism), like rubies and tourmalines, or three colors (trichroism), like andalusites. (You'll only see two colors at a time through the dichroscope as you rotate the rough, though).

• Andalusite — Yes (trichroic)
• Apatite — Yes, weak
• Beryl — Yes, but usually weak
• Chrysoberyl — Yes, but usually weak
• Danburite — Yes, weak
• Diopside — Yes, weak
• Garnet — No (occasionally, a color change garnet may show pleochroic-like colors due to anomalous double refraction)
• Iolite — Yes
• Opal — No
• Orthoclase — No
• Peridot — Yes, but very weak
• Quartz — No
• Sapphire — Yes
• Scapolite — Yes
• Spinel — No (I have seen a couple of color change stones that showed anomalous pleochroic colors)
• Sphene — Yes
• Spodumene — Yes
• Tanzanite — Yes
• Tourmaline — Yes
• Topaz — Yes
• Zircon —Yes

Keep in mind that some doubly refractive gems may not show any pleochroic colors. (Sometimes, the color difference is too slight for our eyes to perceive). Some singly refractive gems may show pleochroic-like colors due to crystal strain. However, you can still use this information as a general guideline for quick separations.