The spectroscope is one of the gemologist’s most useful and powerful tools. Even if it requires a little adaptation time for its use, this quite relative drawback is very largely offset by its strengths.
The spectroscope uses the principle of diffraction. It breaks down the light into rainbow colors, like the diagram below.
The composition of the gem to be analyzed implies that not all light rays are transmitted. Some are absorbed and others are “boosted”. The absorbed colors appear dark, this is called absorption lines or bands. The boosted colors are called emission lines.
Before we see how to use them, let’s first see the two types of spectroscopes and their advantages / disadvantages.
The Diffraction Grating spectroscope or diffraction spectroscope is a tube at the end of which is placed a filter with a multitude of vertical engravings which separates a light ray into all its components of rainbow colors. This is called a spectrum. The advantage of this spectroscope is that the wavelengths are evenly distributed across the width of the spectrum, which makes it easy to read. It is the perfect model to start! The flip side is that this spectroscope renders an image darker than its prism counterpart, which requires good lighting of the stone.
The prism spectroscope has the same shape as the Diffraction Grating spectroscope, except that the filter is replaced by a prism. This has the consequence of making the spectrum much brighter, but also of modifying the scale: the longest wavelengths (red) are narrower than the shortest wavelengths (purple). The side of the red-orange spectrum is therefore more compact than the blue-violet side. It is difficult under these conditions to estimate the wavelength of an absorption or emission line. Note that on some more sophisticated models, a wavelength scale is included (see the Kruss model in the first photo on this page) To appear, the scale must have its own light source, which makes this spectroscope little practical use in the field and limit it to the laboratory, not to mention the fact that it is more fragile and much more expensive!
The use of the spectroscope implies having a light presenting the whole of the light spectrum. Indeed, using a light without yellow, for example, would give a spectrum with an absorption band of yellow, which would be attributable to the light source and not to the stone! LED or low-consumption bulbs have holes in their spectrum. By observing the sunlight directly, we realize that the spectrum of the sun also has absorption lines. So this is not a solution. The only alternative is to use a lamp with a mini Maglite with incandescent bulb (no DEL!!). The spectrum is a bit bright in the red, but at least it’s full. The absorption or emission lines will therefore come from the stone only.
Once these precautions have been taken, it suffices to align the light source, the stone and the spectroscope to start the observations. There are three types of elements visible in a spectroscope: absorption lines, absorption bands and emission lines.
Most chemical elements absorb certain wavelengths of the visible spectrum. When these wavelengths are narrow, we observe fine lines (the small black lines n ° 1). When the wavelengths are numerous and stuck together, it is a dark band that we observe (n ° 2). Finally, when the absorbed wavelengths return energy, we can sometimes see emission lines (the highlighted lines n ° 3)
Each spectrum is typical of a gem. Certain spectra are characteristic, such as that of zircon which sometimes presents a network spectrum, with a multitude of absorption lines.
A wide variety of gems must be observed in order to retain spectra, as amateur spectroscopes generally do not have a graduated scale. Either way, in the field it is much faster and more convenient to use a spectroscope without a scale.
The spectroscope is especially useful for quickly separating visually close gems like spinel and corundum. On the other hand, it generally does not allow to separate natural gems from synthetic gems because they have the same chemical formula. It does not allow the treatments to be identified either.
To go further
gemologist’s loupe
lighting
tweezers
darkfield loupe
polariscope
dichroscope
Chelsea filter and other filters
refractometer
microscope
UV lamp
karat scale