The Index of Refraction, Birefringence and Dispersion are
somewhat exotic properties for ordinary rockhounds, but they are consistent
properties in that minerals never stray far from their known values. This
makes them invaluable tools to mineralogists for identifying and studying
The index of refraction is the
geometric ratio of the angle at which light comes to the crystal (called
the angle of incidence) by the angle at which light is bent as it enters
a crystal (called the angle of refraction). Metallic minerals do not have
an index of refraction because they do not allow light to enter the crystal
in the first place. The laws of refraction (called Snell's Laws)
were laid down by Willebrod Snellius in 1621 and he proposed the following
r) = n
Where i is the angle of incidence
and r is the angle of refraction and
n is the index of refraction. It turns
out that this ratio n is also the ratio
of the speed of light in air to the speed of light in the crystal. This
relationship shows the impact of density or
to the index of refraction in that the greater the density
the slower the speed of light. But density is not the only impact to the
index of refraction (if it were, we could used index of refraction to measure
density and we can't do that, directly anyway) as chemistry and structure
play an important part too. Generally the index of refraction for minerals
falls between 1.4 to 2.0 with a few exceptional mineral exceeding 2.5.
The symmetry of
the crystal has interesting impacts to the index of refraction. Isometric
and amorphous minerals have essentially the same structure or lack there
of, in all directions and so have only one index of refraction and are
called isotropic minerals. But hexagonal,
trigonal and tetragonal
minerals have a different structure along their primary axes than they
do in all other directions and for this reason they have two indices of
refraction; one along the primary axis and one for every other direction.
These minerals are called uniaxial minerals for their one unique
monoclinic and triclinic
minerals have two planes of equal refractive indices and are called biaxial.
The average collector might be able to use the index of refraction to
gauge a mineral's sparkle and generally
that have a high index of refraction are desired above others. Gemstones
that have an index or refraction near 2.0 or higher are considered good
refractive stones. Observe the different gemstones above for the ones with
high index of refractions and those with low values. The following properties
of birefringence and dispersion are closely related to the index of refraction.
AND DOUBLE REFRACTION
The difference between the highest and lowest index
of refraction in a mineral is called the birefringence.
The birefringence is generally low in most minerals but is high for carbonates
and a few other minerals. Calcite
has one of the highest degrees of birefringence and this causes the phenomenon
of double refraction. Double refraction occurs when a ray of light enters
the calcite crystal and due to calcite's high birefringence, the ray is
split into beams, one very fast and one very slow; relatively that is.
As these two beams exit the crystal, they are bent into two different angles
(the angles of refraction ) because
the angle is directly affected by the speed of the beams. A person viewing
into the crystal will see two images ..... of everything. The best way
to view the double refraction is by placing the crystal on a straight line
or printed word (the result will be two lines or two words). There is only
one direction that the beams are both the same speed and that is parallel
to the C-axis or primary trigonal
axis. Rotation of the crystal will reveal the direction in the crystal
that is parallel to the C-axis when the line or word becomes whole again.
By contrast, the direction perpendicular to the C-axis will have the greatest
Dispersion is more of a concern to gemologists than to mineralogists.
It is a very important property to used to identify and qualify
Dispersion is another property that is affected by the index of refraction.
The above discussion of refraction dealt with
the refraction of only the same wavelength of light. But to make it more
complex, refraction is affected by the wavelength as well. Blue light is
bent more than green light which is bent more than red light. If dispersion
in a mineral is low, than white light can travel through the mineral nearly
unaffected and emerge as white light. But if dispersion is high, the white
light will have its component wavelengths or colors dispersed through increasing
refraction. This is what causes the flashes of color, called fire,
in cut gemstones. Diamond
is the champion of cut stones and has a high degree of dispersion or fire
that is almost always unmatched by diamond simulants. Zircon,
cubic zirconia and YAG all have high dispersions and are the popular diamond
impostors of the day, although zircon is a lovely gemstone in its own right.
Dispersion is the reason we have rainbows and why a glass prism can separate
light into its many colors.
Technical Crystal Habits
Descriptive Crystal Habits
Index of Refraction
Reaction to acids