Several important optical properties are applicable to minerals and
gemstones, and can be very useful for gem
identification. With proper equipment, jewelers can easily distinguish a
Ruby from Garnet or red glass, even if their
outward appearance may be identical.
White Light, or visible light, is a form of electromagnetic radiation
(energy waves produced by the motion of an electric charge). White light belongs to the
color spectrum, which defines all
forms of light and electromagnetic radiation. The spectrum also includes many forms of
light not visible to the human eye, such as ultraviolet
and infrared light.
The rate of motion produced by the electrical charge
defines the wavelength of light. Different wavelengths produce different types of light;
white light encompasses all wavelengths visible to the human eye. White light contains the
seven primary colors: red, orange, yellow, green, blue, indigo, and violet.
The color white is a composite of all colors. Every
substance receives its color from the way white light reacts to it. Light can either be
absorbed into a substance, or it can be reflected. The presence of certain elements or
chemicals in a substance determine which wavelengths (i.e. colors) are reflected and which
are absorbed. The wavelengths reflected off a substance determine its color. For example,
if a substance absorbs all wavelengths except for yellow and green wavelengths, which are
reflected, the color of the substance is yellow-green. If a substance absorbs no
wavelengths but reflects them all, its color is white. If, however, a substance absorbs
all wavelengths, its color is black.
In addition to reflection and absorption, light can also be
passed through a substance. Light passing through a substance determines its transparency. If all light passes through a
substance, and none is reflected or absorbed, the substance is transparent and colorless.
These three attributes (reflection, absorption, and passing through)
determine the color and transparency of a substance.
Optical properties relevant to minerals
and gems:
- See also "Color" in
mineral properties
REFRACTIVE INDEX (RI)
The speed of light varies in substances. The speed of light is different in air, water, and other dimensions, including
minerals and gemstones. When light travels from one dimension to another dimension, the light bends, or
refracts, upon entering the second dimension. This phenomenon can be witnessed with
a stick protruding from a pond, where the stick appears to "bend" at the water
level. This is caused by the difference in the speed of light in air and the speed of
light in water. How much the light will bend, or the angle of refraction, depends
on difference in the speed of light of the two substances. All transparent gemstones refract
light, since the speed of light is different in the air than in gemstones.
Every gemstone refracts at a distinct, individual angle. The
angle of refraction is directly related the speed of light in the gemstone. The refractive
index of a gemstone measures the difference between the speed of light in air and the speed
of light in the gemstone. This is determined by the gemstone’s angle of refraction. Every gemstone
has a unique refractive index, meaning every gemstone refracts light at a unique angle.
The refractive index value measures how much slower light
travels in the gemstone than in the air. For example, the refractive index of
Diamond is 2.42. This means that
the speed of light in Diamond is 2.42 times slower than the speed of light in air.
Refractive indices of minerals range from 1.2 to about 3. However, gemstones with a refractive index
greater than Diamond (2.42) are either synthetic or are too soft for practical gemstone use.
The greater the refractive index of a gemstone, the more brilliant or lustrous it is.
The refractive index of a gemstone is measured with a refractometer,
a tool that measures angle of refraction. This tool is used by almost all gemologists and
professional jewelers, for it provides simple, inexpensive, and accurate gem identification. However, a
refractometer cannot read values greater than 1.86. Gemstones with refractive indices greater
than 1.86 can only be tested by placing them in a liquid with a known refractive index,
and then calculating the difference in refraction between the liquid and the gem.
DOUBLE REFRACTION (DR)
Another optical property, known as double refraction
or birefringence, is present in all non-amorphous minerals that do not crystallize
in the isometric crystal system. When
light rays enter birefringent minerals (minerals with double refraction), the light
divides into two rays. The two rays differ in their angle of refraction. Therefore, all
birefringent minerals have two refractive indices, one for each ray. The double refraction
in most minerals is so weak that it cannot be observed with the naked eye. However, a
small number of minerals have a strong double refraction, which is easily seen when the
crystal placed over an image appears to "double" the image.
Double refraction is an important guide to gem
identification. When viewed through a refractometer,
birefringent minerals show two readings – one for each refracted ray of light. Double
refraction is a characteristic trait, meaning every specimen of the same gem always has
the same double refraction.
Double refraction is measured by the difference of
refraction in each light ray. For example, if a gem is placed in a refractometer and shows
a double reading of 1.62 and 1.63, its double refraction is .01.
Highly birefringent gemstones, such as Zircon, must be cut in a way where
the double refraction is least noticeable in the finished gem, for it otherwise appears
blurry.
DISPERSION & FIRE
Dispersion is the splitting of white light into the
colors of the spectrum. This effect is observable in faceted, transparent, colorless gems,
where the white light disperses in the gem and reflects on its inner surfaces,
giving the gem a colorful sparkle. This effect is known as fire in the gem trade.
Generally, gems with higher refractive indices display greater fire. Diamond has the
greatest fire of all true, non-synthetic gems. Room lighting conditions play an important
role in fire, for the stronger the light, the more intense the fire appears to be.
The design of the brilliant cut was extensively
researched to offer a gemstone its maximum amount of fire. For this reason, many transparent gems
with high dispersion are faceted with the brilliant cut.
ABSORPTION SPECTRUM
The absorption spectrum describes the spectral
wavelengths absorbed by a gem. The chemical structure of each gem allows only certain wavelengths to be absorbed (the rest are
reflected or pass through). The wavelengths absorbed into a gem can be detected with an
instrument known as a spectroscope. Color alone cannot be used to identify a gem,
for many gems have identical colors. A spectroscope examines the "true" color of
a gem. Examining the absorption spectrum of a gem is one of the most useful and practical
methods of gem identification.