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
- What it is: An optical instrument containing a small crystal of calcite (Icelandic spar). When light from a gemstone passes through the crystal, the light is split into two polarized beams.
- Application: The instrument is used to detect pleochroism (or dichroism), which means that a stone shows different colors depending on the angle from which you look at it. This depends on the crystal structure of the mineral.
- Effect: When you look into the dichroscope, you will see two small windows side by side. If the stone is birefringent, the two windows will show different colors or shades of color. If the stone is monorefractive (such as diamond, garnet or glass), you will see the same color in both windows. It is an excellent tool for, for example, distinguishing a genuine ruby ??(dichroic) from a red glass imitation or garnet (non-dichroic).
