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.
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