Value
of a gem dependa mainly on its
optical properties such as Color,
Clarity, Transparency and Dispersion
etc. in addition to this there
are some special optical effects
which increase the value of
a gem.
Fire
Fire
is the sparkle of a Gemstone,
when white light splits in to
rainbow colors it shows fire.
The greater the spectrum of
colors the greater is the fire.
The Gems with higher refractive
indices and higher dispersion
values show higher lustre. Diamons
has a higher RI and higher dispersion
value than any other natural
colorless transparent stone.
Chatoyancy
Chatoyancy
is the Catseye effect in gemstone.
It is a reflection which appears
as bright band of light across
the surface of certain gemstone,
when cut en cabochon in certain
orientation, chatoyancy is caused
by reflection from oriented
long inclusions when these inclusions
are parellel to one direction.
Inclusions are needles shaped
parallely arranged fibers of
other mineral substance or tube
like cavities.
Chatoancy
is derived from the word 'chat'
(cat), 'oeil' (eye), catseye
The
stones which show this effect
;
Chrysoberyl, Fibrolite, Tourmaline,
Moonstone, Beryl, Konerupine,
Quartz, Scapolite, Diopside,
Alexandrite, Apatite.
Chrysoberyl
- Because of tube light cavities
or short needles in direction
parellel to the vertical axis
of a crystal. "Cymophane"
name applies to chrysoberyl
catseye only.
Other varieties of gems with
chatoancy prefixed with the
species name.
Ex
: Quartz catseye
Tigers eye - Silicified crocidolite,
oxidized to brown color which
keeps the fiber and structure.
Hawks eye - Fiberous structure
of Crocidolite mineral gives
blue grey to blue green colors
and the catseye effects.
Asterism
The star effect of the Gemstones
is called Asterism. It is caused
by reflection from two more
sets of parallel fibrous or
channel inclusions oriented
with in a gemstone producing
a star. These inclusions running
in direction parallel to inclusions
prism faces of the crystals.
When
light is reflected from incluions
with in the stone called "Epiasterism".
When light is transmitted through
the stone and illuminated the
inclusions called "Disterism"
when cut en cabochon at right
angle to vertical crystal axis
parellel to the basal plane.
It shows some streaks up stars.
The gemstone must be cut as
a cabochon that is with a smoothly
polished domed top, So that
the incident light is reflected
from the inclusions or cavities.
The reflections take the from
of two or more lines depending
on the number of sets of inclusions.
Gemstones that show this effect
are, corundum, Almandine Garnet,
Spinel Quartz, Beryl, Corundum
normally shows six rayed star
or 12 rayed.
Garnet and Spinel 04 rayed or
six rayed stars
Quartz 06 rayed star
Dioside 04 rayed star
Metamerism
The
change of body color according
to the change of one type of
illumination. It is because
of the selective absorption
of light rays differently form
two type of illuminations is
has different is spectral power
distribution.
The
alexandrite variety of chrysoberyl
is the best example for this
effect. Alexandrite shows red
color under tungsten light which
has more red and yellow rays.
And shows green under day light
or flouresent light which has
higher spectral power of blue
rays.
The
color changing percentage of
alexandrite depends on the chromium
percentage of it.
Eg. The stones which display
color change effect are Garnet,
Alexandrite, corundum (Natural
and Synthetic) Spinel, Badeite,
Fluorite.
Some
strongly pleochroic stones such
as andalusite show false color
change effect.
Aventuriscence
Aventuriscence
is a type of iridescence form
some kind of mineral inclusions,
as flakes. Usually this type
of stones are cut en cabochon.
The
stone which show aventuriscence
;
