Colored Stones : Optical Properties

Introduction:

Of all the various properties of a gemstone the optical characteristics are of unsurpassed importance. Optical properties are important because they provide a nondestructive means for identifying gemstones and are responsible for all the features one immediately observes and admires including color, luster, brilliance, scintillation, and dispersion, as well as special phenomena, play of colors, labradorescence, and the like. Brilliance and scintillation are the sparkle and flashes of white light emitted from the gemstone, while dispersion or dispersive refraction is the break up of white light into component spectral colors or the fire of a gemstone. Ultimately, it is the crystalline structure and chemical composition of gem materials that affect the behavior of light and are responsible for optical properties of gemstones. However, it is the responsibility of humans to fashion the rough to maximize these characteristics of unsurpassed importance.

Luminescence/ Fluorescence

Gems that emit visible light after exposure to short-wave or long-wave ultraviolet radiation, they are said to be luminescent, or more specifically, flourescent. A gem is phosphorescent if the luminescence continues after the UV light source has been removed. This phenomenon of fluorescence and/or phosphorescence is the result of UV radiation absorbed by impurities or structural defects within the crystal structure, resulting in an oscillation of electrons between energy levels, and transmission of visible light.

The fluorescence can be a bold green, orangish-red, or whitish blue, and vary in intensity. Fluorescence may be unpredictable because some gems will have no reaction to the UV light source. Fluorescence testing involves cleaning the stone to be tested and then locating a very dark area for observations. Never look at the UV light source directly, as permanent damage to your eyes can occur. Place the gem on a surface when testing (not between metalllic tweezers or your fingers). Stones that look purplish are inert, with the "purple" color being a reflection of the light source.

Luster:

The luster or brilliance of transparent gems is caused by light reflecting from the stone's surface. The smoother and more highly polished the surface is, the greater the luster will be. High light refractivity of a gem will cause greater luster as well. The most intensive luster is seen in the highest refractive indices, diamond, zircon, and rutile, and is known as an adamantine luster. Hematite produces a metallic luster, even though it is not transparent. Most gemstones have a vitreous or glassy luster, but there are other types of lusters, including resinous (amber), greasy (serpentine), waxy (turquoise), pearly (rhodonite), and silky (tiger's eye).

Refraction and Birefringence:

When a ray of light passes from air into a denser medium, such as a gemstone, part is reflected from the surface and part enters the gem material. Light entering the gem is slowed and bent, with the amount of bending dependent upon the angle with which it hit the surface and velocity of light in the two media. Higher angles and greater velocity difference between air and the gem will result in greater refraction. This refraction or bending of light can be measured and this number is termed the refractive index. This index is a constant in different types of gems and is used in identification. The refractive index is defined as a ratio of the speed of light in air to the speed of light in the stone. That is, if the speed of light in air is 300,000 km/sec and speed of light in diamond, 125,000 km/sec, then dividing 300,000 by 125,000 is 2.4 or the refractive index of diamond is 2.4. Light in air is 2.4 times faster than the speed of light in diamond.

When light hitting a gemstone splits into two rays traveling through the stone at different speeds and in different directions, the reaction is called birefringence or double refraction. This is seen uncommonly and in a variety of calcite called Iceland Spar as well as zircon, rutile, and sphene.

All transparent substances can be classified as either isotropic or anisotropic. Isotropic includes amorphous mineraloids and minerals in the isometric crystal system. Light entering isotropic gems moves in all directions with equal velocity, creating only one index of refraction. That is, a single refractive index results when light is considered moving in a wave motion with vibrations in all directions at right angles to the direction of propagation. Put another way, imagine snapping a rope and seeing a wave traveling from one end to the other, with motion perpendicular to the rope (e.g., side to side or up and down). The wave travels in a single direction but is free to vibrate in random directions perpendicular to this single direction of travel. Hence, single refraction is the optical characteristic of light passing through a denser medium without polarization.

In contrast, when light enters anisotropic gem materials the light is split into two polarizing rays, vibrating in mutually perpendicular planes. Thus in a given orientation, two refractive indices, one associated with each polarized ray, is detected and the specimen is termed doubly refractive. Double refraction occurs in specimens from five of the six crystal systems, including tetragonal, orthorhombic, hexagonal, monoclinic, and triclinic. These anisotropic minerals possess the power to polarize light or confine the light wave to vibrate in only one direction, blocking all other waves and spliting light into two rays that travel at different speeds at right angles to one another.

Dispersion :

Light is slowed and refracted or bent upon entering a denser medium. A characteristic refraction or bending is associated with each different wavelength of light and this separation of white light into component colors is called dispersion. In other words, dispersion is the separation of light into its separate spectral colors.

White light is in fact a combination of red, blue, and green wavelengths of light. Recognition of this was credited to Sir Isaac Newton in 1666 when he observed a dispersed spectrum through a glass prism. Although he recognized the colors as a continuum, he assigned seven names, violet, indigo, blue, green, yellow, orange, and red by analogy with seven notes in a musical scale. Newton's observation of dispersion or dispersive refraction demonstrated that a light beam bends or refracts as it passes from one medium to another, such as from glass to air or from gem materials to air.

So, while dispersion through water drops in atmosphere produces rainbows, the same rainbow effect occurs in diamond. In addition, the wavelength of light will determine the amount of refraction or bending; shorter wavelengths in the blue end of the spectrum bend more than longer wavelengths in the red end. Dispersion is actually measured by subtracting the refractive index of red light from the refractive index of violet light. Diamond's dispersions is 0.044 while quartz is 0.013; diamond is more dispersive than quartz.

Note:

Gemstones with the highest light refraction typically show the highest dispersion rate as well (rutile, sphene, diamond, zircon). This color dispersion or fire can be enhanced by a gem cutter if he uses an appropriate facetting style.

Pleochroism:

Light that passes through a doubly refractive gemstone or anisotropic mineral is split in different directions with varying velocity. The light is absorbed differently in different vibration directions, resulting in color variation known as pleochroism e.g. iolite, alexandrite and andalusite.

In minerals with only two rays, two pleochroic colors can be detected, called dichroism. In minerals with three principal vibration directions (three refractive indices), three different pleochroic colors can be detected, called trichroism (with only two observed in any one direction). It is very important for the gem cutter to cut a pleochroic stone properly in order to show off the different colors.

In order to see pleochroism, the gem must be colored (colorless gems transmit all colors of the spectrum of white light), a single crystal (an aggregate of crystals would scatter the light, obscuring the pleochroism), fairly transparent (numerous inclusions would again scatter the light), and viewed in some direction other than parallel to an optic axis. Ruby and sapphire have two color shades and are pleochroic; in ruby, for example, yellow-red and purplish-red, which distinguishes it from garnet and red spinel, which have no pleochroicism.