The dichroscope is a pocket-sized tool that 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 used to inspect pleochroism in gemstones and to quickly separate some common stones from each other (such as ruby versus red garnet).
There are two types of dichroscopes on the market:
- calcite dichroscopes
- polarizing dichroscopes
In anisotropic gemstones, different colors are absorbed in different directions ("directional selective absorption") which causes the pleochroism that is observed with the dichroscope.
The calcite dichroscope is the preferred type of dichroscope used in gemology. It works because the calcite rhomb (Iceland spar) separates the polarized slow and fast ray emerging from the gemstone. If you look at the viewing end of the dichroscope, two small windows are seen.
A gemstone is placed in front of the aperture (slightly touching it) with a strong white light source (such as a penlight) directly behind the stone. Light enters through the aperture of the dichroscope. The pleochroism colors inside the gemstone are separated by the calcite rhomb. The glass prisms on either side of the rhomb are there to guide the light straight through the instrument.
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 a 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 clear 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
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 each other 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 while keeping the stone in a fixed position and observe if the colors in the two windows are the same or if they differ.
This procedure should be repeated another 4 times from different angles.
- through the table
- through the crown facets
- another direction through the crown facets
- through the girdle
- a different direction through the girdle
Of course for en-cabochon cut stones there are no tables or 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.
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, such as heated citrine.
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 exist. Low type zircon may show up as isotropic, while peridot (which is biaxial) may appear uniaxial.
These stones are considered "dichroic" (di = 2, chrom = colored).
When in total 3 colors (or shades of color) are seen after 5 readings, the stone is 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, chrom = colored).
- Gem Identification Made easy 3rd edition (2006) - A.C. Bonanno, Antoinette Matlins ISBN 0943763347
- Gemmology 3rd edition (2005) - Peter Read
- Guide to Affordable Gemology (2001) - Dr. W. Wm. Hanneman
- Ruby & Sapphire (1997) - Richard W. Hughes ISBN 0964509768