24.04.2023 DISPERSION OF PRECIOUS STONES
When turned in the light, some gemstones display a fascinating play of colors: thousands of points of light in the colors of the rainbow flash and dance around the jewel with each turn. This optical phenomenon is called “dispersion.” In gemology, it is known as “fire” and is considered a quality characteristic of gemstones.
WHAT IS DISPERSION?
The term “dispersion” comes from physics and describes how incident light creates a color spectrum through refraction in a medium.
It is well known that light, although it appears white or colorless to our eyes, consists of different colors. When it is broken down into its components as it passes through an object, the spectral colors red, orange, yellow, green, blue and violet appear. In pop culture, this optical effect is best known as the motif on the record cover of the Pink Floyd album “The Dark Side of the Moon.
The respective color is determined by the wavelength of the light: For example, we perceive light with a wavelength of 570 to 600 nanometers as yellow. Since light of different wavelengths or colors has a different propagation speed, either stronger or weaker refraction occurs. This causes the different components of white light to fan out into a spectrum. This color spectrum comprises the wave range that is visible to the human eye – the spectral colors.
DISPERSION: THE FIRE OF PRECIOUS STONES
In gemology, the dispersion of gemstones is called “fire”. The more fire a stone has, the higher its value is classified. In contrast, the dispersion must be as low as possible for optical lenses in eyeglasses or photographic lenses, otherwise disturbing color fringes will appear.
The fire is particularly strong in transparent, colorless and cut gemstones with many facets. That is why colorless brilliant cut diamonds are known for their fire. In colored gemstones, their inherent color provides a distortion of the colors produced by the refraction of light. Opaque minerals do not exhibit dispersion.
However, the colors produced by the dispersion of light depend not only on the structure of the gemstone, but also on the frequency of the incident light and the speed at which the light propagates through the material.
The decisive factor for impressive dispersion in a gemstone is the cut. With extreme precision, the facets – the cut surfaces – are carved out to enhance the fire. However, the setting of a gemstone also contributes to the fire by giving it additional shades of color.
MEASUREMENT OF THE DISPERSION OF PRECIOUS STONES
To measure the dispersion of gemstones, the refraction of long-wave red light is compared with the refraction of short-wave violet light. For this purpose, the Fraun’hofers lines B (red region of the color spectrum) and G (violet region of the color spectrum) are used (= BG dispersion). Sometimes the value of the CF dispersion (orange and blue-green color spectrum) is also determined.
The dispersion is measured with a refractometer, which determines the refractive index of a gemstone. Each mineral has an individual dispersion, because the wavelengths of light are refracted differently by each stone. For this reason, dispersion is also suitable for determining gemstones.
Although diamond in particular (BG value of 0.044) is known for its sparkle in the colors of the rainbow, it is not the gemstone with the highest dispersion value. Rutile (0.280), zirconia (0.066), demantoid (0.057), and titanite (0.051) have particularly strong BG dispersion. However, gemstones such as Tanzanite (0.030), Peridot (0.020) and Ruby (0.018) are also characterized by their pronounced fire. Jade, turquoise and lapis lazuli, on the other hand, show no dispersion.