Gemstone Color

Color is the apparent result of selective absorption or transmission of
different frequencies of visible light. Color can be described as the
combination of three characteristics: hue, tone, and intensity. Hue is a
function of the frequency of light and is described by familiar terms such
as red, orange, yellow, blue, green, indigo, and violet. Tone is a variation
from very light to very dark. Intensity is a measure of saturation, or
purity, of a color. The typical human eye can identify approximately 150
pure hues, but around one million colors. The differences among colors may
be immediately obvious or so subtle that direct comparison under controlled
conditions is required to discern them. Color acuity is also highly affected
by fatigue, diet, and other factors, so it is unwise to attempt judging
subtle color differences in gemstones such as diamond without comparison
stones and careful control of lighting conditions.

Pleochroism is the apparent change in color of a doubly refractive gemstone
when viewed through different directions of the crystal structure. In most
cases, the color variations are not obvious to the unaided eye and must be
viewed through a polariscope or dichroscope, but in some cases, the
pleochroic colors are strikingly obvious. For example, many green
tourmalines appear black through the C axis of the crystal, and iolite shows
a striking combination of blue-violet and near colorless. Dichroism refers
to the display of two ("di") pleochroic colors in a gemstone.

Alexandrite-like color change, or photochroism, is the marked change in
perceived color of a gemstone under different lighting conditions. As the
name implies, the most famous example appears in alexandrite, a form of
chrysoberyl that typically appears blue or green in daylight and red or
purplish in incandescent light, but similar color changes may be observed in
sapphire, garnet, and tourmaline. The phenomenon is due to selective
absortion of different wavelengths of light, and the predominance or absence
of those wavelengths in the prevailing light (incandescent light has
proportionately higher quantities of reddish wavelengths and less of blue or
green).

Optic Character

Gemstones may affect the passage of light differently through different
directions in the crystal structure. If the velocity of light is constant
through all directions in the stone, the stone is said to be singly
refractive, or isochroic, and has one refractive index. This is
characteristic of isometric crystals. If the velocity of light varies with
direction, the stone is doubly refractive, or anisotropic, and has two
refractive indices. In anisotropic materials, light is separated into two
polarized components, the ordinary ray and the extraordinary ray.
Anisotropic materials can be further characterized as uniaxial, biaxial
positive, and biaxial negative.

Amorphous (non-crystalline) materials, such as opal, amber, and glass, may
scatter light in unusual directions due to internal stress and display a
phenomenon known as anomalous double refraction.

Refractive Index

Refractive index, or R.I., is the ratio of the velocity of light in air to
the velocity of light through a transparent material. If light passes from
air into a transparent material at an angle of incidence other than a 90
degree angle, it is deflected at a different angle (the coincident angle)
according to the R.I. Gemstones with higher R.I. are generally more
brilliant than those with low R.I. For example, diamond has an R.I. of about
2.4; quartz, about 1.54-1.55. The R.I. of most gemstones is easily measured
using a simple optical instrument known as a refractometer.

Birefringence

Birefringence is the difference in value between the highest and lowest
refractive indices in a doubly refractive (anisotropic) material. Depending
on the orientation of a faceted stone, this can result in a "fuzzy"
appearance and apparent doubling of facets viewed through the stone.

Dispersion

Dispersion is the ability of a gemstone to separate light into its component
colors; that is, the quality of passing different wavelengths of light at
different velocities. Dispersion is the quality in a diamond that produces
sparkles of color in an otherwise colorless stone. Quartz, which has a
dispersion of 0.013, shows much less of this effect than diamond, which has
a dispersion of 0.044. Diamond, in turn, shows much less color play than
sphalerite, which has a dispersion of 0.156.

Fluorescence

Many materials are fluorescent. That is, when exposed to ultraviolet light
or X-rays, they transform some of the incoming energy into visible light.
The color and intensity of the fluorescence is often indicative, but not
conclusive, of the identity of the material. For example, natural yellow
sapphires from Ceylon show a distinctive apricot-colored fluorescence, while
synthetic yellow sapphires generally show no fluorescence or a dull red when
exposed to long-wave ultraviolet (UV) light. Most natural emeralds are inert
(non-fluorescent) under long-wave UV, and most synthetic emeralds show a
moderate to strong red fluorescence. Because of the prominent exceptions,
this test alone is inconclusive.

Phosphorescence

If a fluorescent material continues to emit light after the exciting UV or
X-ray light is removed, it is said to be phosphorescent. This phenomenon
usually lasts only a few seconds but may occasionally persist for much
longer periods. This is a relatively rare characteristic in gemstones.

Clarity

Gemstones can vary from complete opacity to lucid clarity and may contain
few or many inclusions such as crystals of other minerals, gas- or
liquid-filled cavities, or even insects! (Large, perfectly preserved insect
specimens in amber are highly prized.) In some gemstones, such as emerald,
certain inclusions are highly distinctive and can be used as reliable
indicators of identity. A gemological microscope (a binocular microscope
with a typical magnification of 10X to 40X) is one of the most useful tools
in identifying many gemstones, as well as grading them on relative clarity.

Specific gravity

Gem materials vary greatly in density -- amber may float in salt water
(density near that of water), while hematite is more than five times the
density of water. This is why two different gemstones may have the same size
but different weights and vice versa -- a one carat round brilliant diamond
of typical proportions will be approximately 6.5 mm in diameter, while a
round brilliant ruby of the same size (6.5 mm in diameter) and proportions
will weigh approximately 1.55 carats. Generally, gemologists refer to
specific gravity, or relative density -- the ratio of the density of a
gemstone relative to that of water.

Durability

The two most familiar qualities of durability -- hardness and toughness --
are often misunderstood. Hardness is resistance to scratching or piercing.
Toughness is resistance to breakage. The combination of the two largely
defines the durability of a gemstone. Diamond is the hardest naturally
occurring material and is also quite tough; however, it can be broken by a
hard blow. Jadeite and nephrite (the jades) are much softer and relatively
easy to scratch but are perhaps the toughest gem materials. Hardness is
often represented on the Mohs scale, a nonlinear scale of scratch resistance
varying from 1 (talc) to 10 (diamond). The Mohs scale can be misleading --
there is a much greater difference in hardness between 9 (corundum) and 10
(diamond) than between 9 and 1 (talc). More precise, and less familiar,
measurements of hardness are done using other systems, such as the Knoop
scale of resistance to indentation. Because of the likelihood of physical
damage, hardness tests are NOT recommended for gem identification.
Resistance to chemical degradation or to changes in temperature or humidity
are important. Turquoise is often quite porous and can be discolored by
exposure to oils. Opals are heat-sensitive and have a high water content;
sudden temperature changes or extremely dry conditions can cause them to
crack or craze.

Thermal conductivity (the ability to conduct heat) is very low in most
gemstones but is extremely high in diamond (from 1.6 to 4.8 times as great
as in pure silver!). This unusual property of diamond is the basis for
several popular diagnostic probes that are used to distinguish diamond from
its numerous imitations.