Of the total amount of light
that falls upon a material, part is returned or reflected
; the remainder penetrates into it, and crosses it or is
absorbed by it. The reflected part of the light produces
what is termed the " luster" of the material.
The diamonds used as gems are generally colorless or only
faintly colored; it can be taken that all the light that
passes into the stones passes out again. The luster of the
diamond is peculiar to that gem, and is called adamantine
for that reason. It is not found in any other gem, although
zircon and demantoid or olivine have a luster approaching
somewhat to the adamantine.
In gem stones of the same kind and of the same grade of
polish, the luster only varies with the area of the gem
stone exposed to the light, and that it is independent of
the type of cut or of the proportions given to the gem (in
so far as they do not affect the area)
Now a gem, not being in itself a source of light, cannot
shine with other than reflected light. The maximum amount
of light will be given off by the gem if the whole of the
light that strikes it is reflected by the back of the gem,
i.e. by that part hidden by the setting, and sent out into
the air by its front part. The facets of the stone must
therefore be so disposed that no light that enters it is
let out through its back, but that it is wholly reflected.
This result is obtained by having the facets inclined in
such a way that all the light that strikes them does so
at an angle of incidence greater than the critical angle.
The critical angle of diamond is 24° 26'
In a diamond, the amount of light reflected from the surface
is much smaller than that penetrating into the stone ; moreover,
a diamond is practically perfectly transparent, so that
all the light that passes into the stone has to pass out
If a gem is so cut or designed that every ray of light
passing into it follows the best path possible for producing
pleasing effects upon the eye, then the gem is perfectly
The reason for a fire or colorful sparkle of a diamond
The greater the dispersion of a medium, other things being
equal, the greater is the difference between the angles
of refraction of the various colors, and the further separated
do they become. It is to its very high dispersion, the greatest
among all colorless gemstones that the diamond owes its
When a ray of light passes through a well-cut diamond,
it is refracted through a large angle, and consequently
the colors of the spectrum, becoming widely separated, strike
a spectators eye separately, so that at one moment one sees
a ray of vivid blue, at another one of flaming scarlet or
one of shining green, all these colors change incessantly
with the slightest motion.
If you are still unclear about the concepts, please refer
the animation below.
Besides the above details the animation file also contains
the following in detail
- Why does a diamond shine brilliantly ?
- What are the design boundaries to be taken in consideration
for a diamond ?
- Why colored light is given out even though when the
light entering is only white?
- How can diamond be differentiated from its look alike
- How does cut of a diamond influence its sparkle ?
- Complete Total internal reflection with critical angle
explained through animation