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Refractive
Index (RI) is
the characteristic slowing of light as it travels through
a given gem species or substance. The higher the RI,
the greater brilliance and luster is possible. The fact
that each gem has its own RI makes it necessary to facet
each species at slightly different angles and makes
it a useful characteristic in gem identification.
What
is the Index of Refraction?
Light moves at its
maximum possible velocity in the absence of matter.
For example, in space light travels approximately 186,000
miles per second. But the presence of matter slows light
down; or more specifically, the forces which bind atoms
slow light down. The Index of Refraction for any substance
is the ratio of the velocity of light in a vacuum to
its velocity in a given substance. The higher the index,
the slower the velocity. Thus, if we say that a particular
crystal has an index of refraction of 2.000 we mean
that light would pass through a vacuum at twice the
velocity that it can go through that crystal.
Why
do different substances have different refractive indices?
As
previously pointed out, the velocity of light through
any material is retarded by forces binding its atoms
and molecules. Therefore, the degree to which a particular
beam of light is slowed depends both upon the types
of atoms present (the chemical composition) and the
nature of the arrangement of those atoms.
For
example, light travels in water at 3/4 of its velocity
in a vacuum, so we assign a value of 4/3, or 1.333 as
the index of refraction of water. But if we freeze that
water into ice, a different arrangement of the molecules
results and the ice has a refractive index of 1.313.
Or, if we boil the water, yet another arrangement of
the same molecules is produced, which retards the light
very little, and the steam has a refractive index of
1.0002.
In
the case of ice, water and steam, we have examples of
three different states of matter; solid, liquid and
gas. But, even when a single state of matter is considered,
it must be remembered that the identical materials can
often be arranged in different structures, which are
reflected in the differing indices of refraction.
For
example, common pure Quartz crystals have an average
index of 1.548. But if the crystal is melted and then
cooled down to room temperature, the resulting fused
silica-quartz has an index of 1.456. Or the same silicon
dioxide may also exist as tridymite, cristobalite, coesite
or other differing crystal structures, each of which
has a different refractive index, although all have
the identical chemical composition.
Does
light always travel through a given crystal at the same
speed?
No. Most minerals crystallize in an orderly
lattice work that has different types of inter-molecular
bonds in different planes, much like the grain in wood.
The strength of these bonds may be very different in
different planes.
The
form of Calcite known as Iceland Spar presents an outstanding
example of the effect on refractive index in different
optical directions, since a given crystal will have
an index of 1.486 in one direction and 1.658 in another.
Fortunately (except for Rutile) most minerals used as
gems do not vary so widely in different directions.
What
other factors affect the Refractive Index of a mineral?
Temperature
of the specimen influences its index, but if measurements
are made at a normal range of room temperatures, this
effect is so small that it can be neglected.
Impurities
in the specimen have an effect proportionate to their
concentration in the specimen. For example, Opal consists
mainly of a form of silica known as cristobalite with
an index of 1.487. But Opal usually contains about 5
to 10% of water in its structure. Since water has an
index of 1.33, Opals show an index somewhere between
those two values, usually 1.43 to 1.46.
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