Instruments that are very useful and classic aids in gemological identification. They are popular because they are portable, quick to use and provide good guidance during the work. You can find more information about these tools and how to use them on international educational sites such as the International Gem Society.
Here is a brief description of how they work and what they are used for:
Chelsea filter
- What it is: A hand-held color filter (invented in 1934) that lets through very specific wavelengths of light: bright green and red. The rest of the light spectrum is absorbed.
- Application: Originally developed as an "emerald filter" to distinguish natural emeralds from glass imitations and synthetic emeralds. Natural emeralds often contain chromium, which causes them to glow intensely red or pink in the filter, while glass and imitations remain green.
- Things to consider: It is a confirmatory tool rather than absolute proof. It can also help identify certain other colored stones such as synthetic spinels and blue sapphires (by revealing the presence of cobalt or chromium).
Ruby Filter
- What it is: A color filter similar to the Chelsea filter, but specifically designed to filter light so that only stones with a high content of the element chromium are clearly visible.
- Uses: It is used almost exclusively to quickly and easily detect rubies (as well as red spinels and synthetic counterparts).
- Effect: When you look at a genuine ruby ??through the filter, it emits a very intense red glow. It is also a very effective tool for revealing fake doublets and triplets (composite stones).
Dichroscope
There are two types of dichroscopes, and the main difference between the instruments is the technique they use to separate light and detect pleochroism (color shifts) in gemstones. A calcite dichroscope uses birefringent calcite crystals (Icelandic spar), while a London dichroscope (which is a polarizing dichroscope) uses two linear polarization filters.
A clear comparison between the two instruments:
Calcite Dichroscope
- Technology: Uses a transparent calcite prism that splits the light from the gemstone into two polarized beams with different vibration directions.
- Display: You see both color beams simultaneously in each small window. This makes it easy to directly compare the color shades side by side.
- Advantages: Industry standard and the most preferred instrument among gemologists. It is very effective in detecting even very weak color shifts (weak pleochroism).
- Disadvantages: Can be somewhat more expensive to manufacture and the calcite crystal can be sensitive to scratches.
London Dichroscope
- Technology: Uses two perpendicular polarization filters (polarizers) mounted next to each other.
- Display: Because the filters block different light vibrations, in older models you can only see one color at a time. However, in modern London dichroscopes, the filters are placed so close together that you can effectively see two fields at the same time (just like in the calcite variant).
- Advantages: A cheaper and more economical option. It is also easier to use if you want to analyze several stones or larger batches from a distance.
- Disadvantages: Optically somewhat inferior to calcite. It can be much more difficult to detect weak pleochroic colors compared to a calcite crystal.
