Have you ever seen the colors of the rainbow coming from deep within a finely cut gem? That effect is known as gemstone dispersion, or a gemstone’s “fire.” Although diamonds are well known for their dispersion, other gems can exceed their fire, like this stunning cerussite gem
or this faceted rutile.
Although dispersion can be appreciated strictly on an aesthetic level, gemologists can measure a gemstone’s dispersion with a refractometer and use that information to identify it. The higher the measurement, the more dispersion the gem can display. Diamond’s dispersion is measured at .044. Cerussite’s is .055. Rutile’s is .28, more than six times that of diamond.
What Causes Gemstone Dispersion?
Normal white light is composed of a rainbow of colors. Each of those colors is a wavelength of light.
When white light passes through a gemstone, each of its colors travels through it at a different speed. This is literally “dispersion”: the wavelengths previously joined in white light “disperse” and exit the gem separately, creating the rainbow color display. For example, blue is a short wavelength of light and travels slowly through a gem. Red is a long wavelength of light and travels more quickly through a gem than blue.
A gemstone’s density, color, refractive index, and faceting all affect how it disperses light. Faceting plays a critical role. Skilled lapidaries can control how light moves within a gem. They can cut a gemstone so that light enters and strikes the pavilion facets at “critical angles,” causing light to be reflected internally and then back out through the crown facets. This maximizes the fire effect for wearers and admirers.
What are the Differences between Gemstone Dispersion, Birefringence, and Pleochroism?
Although dispersion, birefringence (double refraction), and pleochroism are all effects of light passing through a gemstone, they are distinct phenomena and are affected by different properties of the gem.
Pleochroism and birefringent effects are results of how a gemstone’s crystalline structure polarizes light. Only gemstones with non-cubic crystalline structures may show pleochroism, two or three colors, or birefringent effects like double vision and fuzziness.
The use of the evocative term “fire” to describe dispersion is also a good way, if a bit poetic, to distinguish dispersion from pleochroism. The colors created by dispersion seem to be “active” and appear to emanate from little fires within the gem. Pleochroic colors seem to be “conditions” or qualities of the gem itself (even though you may have to turn the gem to see the different colors).
Some of the gems with the most dazzling dispersion, such as diamonds and cerussite, have no pleochroic effects. (Diamonds have a cubic crystal structure and cerussite is usually colorless). However, there are gemstones, such as sphene (also known as titanite), that are pleochroic and have a high dispersion, though they may not show both properties at the same time.