Diamonds, pearls, sapphires, emeralds, amethysts and the rest of the gem kingdom are extraordinarily pleasing to the eye, as their radiant beauty is melting the hearts of millions of admirers from all corners of the world. But, have you ever thought what makes them as dazzling and beautiful as they are, allowing designers to create the most delicate jewelry?
If you are eager to enrich your jewelry knowledge with some new, crunchy details, you have come to the right place, because in this article, we are going to learn about the optical characteristics of precious stones. More precisely, we will pay attention to their luster, luminescence, dispersion, pleochroism and last but not least, refraction and birefringence.
Otherwise known as ‘brilliance’, the luster property of transparent precious stones is caused by light reflecting from their surface. The more polished the stone’s surface is, the higher the level of luster. Gemstones with a high light refractivity are known to showcase great luster as well. By far, the most intensive luster has been seen in the diamond, the zircon and the rutile, also referred to as ‘adamantine luster’. The majority of precious stones feature glassy or vitreous luster, although there are other brilliance types too, including:
* Waxy Luster –Turquoise
* Resinous Luster – Amber
* Pearly Luster – Rhodonite
* Greasy Luster – Serpentine
* Silky Luster – Tiger’s Eye
Gemstones with the ability to emit visible levels of light after being exposed to long-wave or short-wave UV are known in jewelry as luminescent or more precisely, fluorescent. The ‘luminescence phenomenon’ comes as a result of the ultraviolet radiation absorbed by the impurities of the crystal structure, thus resulting in an electron- oscillation between energy levels that provoke the radiation of the stone’s visible light.
Usually, fluorescence appears as whitish blue, bold green and orangish red. Sometimes, this particular gemstone characteristic can be unpredictable, since there are gems that show no reaction to the ultraviolet light exposure.
The dispersion is probably one of the most delicate properties of precious stones, so we are going to make it easier for you to understand this feature and simply define it as the separation of white light into its spectral colors respectively. Therefore, white light represents a compelling combination of blue, green and red wavelengths of light. Going back in history, this phenomenon was discovered in 1666 by the genius Isaac Newton, who developed his groundbreaking ‘theory of color’ that a prism is decomposing white light into the diverse colors of the visible spectrum. The acclaimed scientist even named these colors by analogy with the seven notes of the musical scale, resulting in: blue, green, orange, yellow, indigo, red and violet.
In other words, dispersion happens when a light beam refracts or bends while passing from one medium to another. In our case, particularly, from the gem’s surface to the air. Another interesting thing to know is that the wavelength of light, actually determines the refraction amount. Hence, shorter wavelengths in the spectrum’s blue end refract (bend) more than longer wavelengths in the spectrum’s red end. Generally, dispersion is measured by subtracting the red-light refractive index from the violet-light refractive index. For instance, the dispersion of the diamond is estimated at 0.044 compared to the quarts whose amount of dispersion is 0.013, meaning that it is less dispersive than the diamond.
In modern jewelry, impressive levels of dispersion and fire are achieved with a combination of gem-cutting expertise and appropriate facetting methods.
Pleochroism, also known as ‘iolite’, ‘andalusite’ and ‘alexandrite’, is that special characteristic of precious stones to appear in different colors and nuances when observed from various angles. This impressive phenomenon is a result of the gem’s ability to absorb different transmitted-light wavelengths, depending on its crystallographic orientations. Thus, the light that goes through the doubly refractive anisotropic mineral or gemstone, splits in various directions with different velocity. This way, the light is absorbed differently by each vibration-direction respectively, which results in splendid color variation or simply said, pleochroism.
To show off this gemstone property at its maximum level, it is of the utmost importance to cut the pleochroic stone properly, so that it can display its spectrum of different colors. However, to see the effect of pleochroism, the precious stone has to be a colored one, since colorless precious stones transmit the colors of the white-light spectrum only. The most appraised pleochroic gemstones in jewelry are sapphire and ruby. Each of these two exceptionally vibrant gemstones features two color shades, like for example, the ruby’s purplish-red and yellow-red, which distinguish this gem from the red spinel and the garnet.
REFRACTION AND BIREFRINGENCE
When a beam of light passes from one medium to another, like for example from the air into a gemstone, part of the light is reflected from the gem’s surface and part enters into the gem. That part of the light that enters the gem is slowed and bent, and the exact level of bending is determined by the angle with which the light has entered the surface, also including the speed of light in the two media. Therefore, the higher the angles and the greater the velocity difference between the air and the gem, the greater the refraction will be. Normally, the refraction index is considered a ratio of the velocity of light in air to the velocity of light in the gem. If we take the diamond as an example, the refraction index will show that light in air is at least two times faster than the speed of light in this precious stone.
The refraction activity is mostly seen in gemstones like the sphene, the rutile and the zircon and it is simply described as the process of light travelling through the gemstone in different directions and at different speeds. Refraction can be single and double. Single refraction is the optical property of light passing through the gemstone’s surface without polarization, while at double refraction, the light splits into two polarizing ways.
Written by: Liljana Tomova