Dispersion is an optical property of some gemstones that produces a rainbow sparkle effect. It is this property of gemstones that gives the brilliance to Diamonds. While almost every gemstone can show dispersion, most of the time it is not visible to our eyes. Diamonds are one gemstone that has benefited greatly from the visual feast of dispersion. Diamonds by themselves are white and colorless but when a ray of light hits them they come to life with a sparkle of rainbow colors. Other lesser known gemstones like Sphene have a body color of their own but also show rainbow colors due to dispersion.
Dispersion – The “Simple” Explanation
For the benefit of readers who neither understand gemstones nor the passage of light through them, we will put things into a simplistic explanation. White light enters a gem and then depending on how the stone is cut, it will bounce around and exit the stone. When the light bounces around it is split into the rainbow colors. This is dispersion. Ok, that’s the “in and out” of it, pun intended.
The Not-So-Simple Explanation
The white light that enters the gemstone is made up of a broad spectrum of wavelengths (or colors). Long wavelengths lie at the red end of the spectrum while colors such as blue and violet have shorter wavelengths and lie at the opposite end of the spectrum. Remember the way to recall the colors of a rainbow? Roy G. Biv (Red-Orange-Yellow-Green-Blue-Indigo-Violet)? Well, Roy is “white light”. Pretty cool, right? Back to the light entering the gemstone. Depending on a number of factors like stone density, angle of entrance, angle of the reflective surface or facet, the wavelengths which make up white light will bend before exiting the stone as their associated colors. That sums up gemstone dispersion or the bending of white light as it enters the stone.
Dispersion – The Fire
So how do we get a gemstone to really produce an amazing display of color? If you’re a diamond owner, you already observe, if not know, a great deal about dispersion. So, let’s say you’re not wearing a diamond but another gemstone. Gems are cut to different shapes with variations intended to take advantage of the light. Depending on the cut of the stones and the inclination of those surfaces referred to earlier as facets, light can be directed inside the stone and bent to emanate the rainbow of colors which make up white light.
Can Dispersion Be Measured?
Dispersion can be measured and is usually shown as the difference between the refractive index of Red light and Violet light. The technical term for this is the BG Fraunhofer lines. As an example a Diamond is 0.044 while Sphene is 0.051 and Demantoid Garnet is 0.057.
Take note though that a higher dispersion number does not always mean more visible fire to the eye. Some stones have a darker body color which makes some of the rainbow colors hard to see. Also, some stones with a high dispersion number may not be cut perfectly like a Diamond usually is and so the rainbow may not be visible.
What Gemstones Have Dispersion?
There are many gemstones that have dispersion but most of them are not visible to the naked eye. Below is a list of the mot common ones with the dispersion number listed
- Sphalerite 0.156
- Demantoid Garnet 0.057
- Sphene 0.051
- Diamond 0.044
- Zircon 0.039