Asterism in Gems @ Jewel Info 4 U
By: Nasrina Parvin Khan
Summary: "Asterism" is an exclusive light effect (which we called special optical phenomenon) of some crystal. When light pass through these crystals, its exhibit a star-shaped figure, which called "asterism".
Asterism
Asterism" is a term, which is
most commonly used in gemology. It
refers a kind of special optic
phenomenon that has been seen in
several gems.
Special optic phenomena refer
uncommon and excellent optic
properties exhibit by some rare
gemstone. The phenomena are
particular property of transmitting
or reflecting light. The structure
or inclusions of a mineral in gems
is the reason of this property. The
beauty and value of gems depend on
their optical properties.
"Asterism"-one of the important
optical phenomena has been seen in
sapphires, rubies, rose quartz and
some other gems. The term "Asterism"
came from the Greek word "aster",
which means star. From the name we
can guess "asterism" is star-shaped
light effect across the surface of
gems.
"Asterism" is an exclusive light
effect (which we called special
optical phenomenon) of some crystal.
When light pass through these
crystals, its exhibit a star-shaped
figure, which called "asterism".
This figure is produced by reflected
or transmitted light.Asterism in Gems
Gems, which show an illuminated
star, just bellow the surface of
crystal called star gems. The most
popular star gems are star ruby and
star sapphire. Other gems, which
also possess the special optic
phenomenon "asterism", are rose
quartz,
garnet,
spinel,
Chrysoberyl
and
Diopside. There are a few
specimen of rock crystal, which has
the phenomenon.
The star in the star gems may have
four to twelve rays depending on the
crystal system of the gems. But the
six- rayed star effect is most
common. Usually, six-rayed star show
by the star rubies and star
sapphires. Sometimes these star gems
exhibit the twelve-rayed star
effect. But the twelve-rayed star
effect in corundum is occasional.
Rose quartz exhibits six-rayed star.
Some garnets and spinel commonly
have the star effect with four rays,
but these star gems can show
six-rayed star effect if cut in a
distinct way. Some crystals like
Chrysoberyl and Diopside only
exhibit four-rayed star effect.
"Asterism" is caused by the dense
inclusions of tiny fibres of foreign
minerals in gems. Generally, these
inclusions are the reason of the
reflection of light and form a wavy
star like formation, which moves
around when the mineral is rotated.
All the inclusions in the gems are
formed via exsolution.
The inclusions must be long, very
thin needle like which called as
rutile. "Asterism" is produced when
needles intersect each other in
multiple groups. Small needles of
rutile (titanium oxide), sometimes
called "silk" are oriented inside
the minerals in the direction of the
crystal growth system. There is a
relationship between the gem's
growth and the number of rays
produced by the reflected light.
There must be three or more
chatoyant bands inside the gems to
produce "asterism". The arrangement
of needles in a hexagonal pattern
produced six-rayed "asterism".
Exsolved inclusions form a pattern
in the host mineral. The pattern may
vary from mineral to mineral, but in
case of specific mineral the pattern
remain same. In case of corundum, we
can see rutile is exsolved in three
directions crossing at 60 degree or
120 degree in the basal and produce
six or twelve-rayed star. When the
rutiles crossed at 90 degree, it
produces a four-rayed star.
.
Star gems are translucent to opaque
.The rays of star should cross at
the top of these gems and be
distinct.
Star Sapphire
The sharpness and the size of
"asterism" in gems, which identified
as phenomenon, depend on the quality
and quantity of rutile needles.
Small to smaller size of rutile
needles help "asterism" to be
sharper. When the needles are
visible with open eyes as
inclusions, the gems turned into a
low translucent stone and lack
"asterism".
Large quantities of rutile needles
in a crystal produce phenomenal
"aster". If the quantities of
rutiles are small,
"Asterism" is visible in only a
small area. These gems are not
recognized as phenomenal gems .The
clarity of these gems can be improve
by dissolving some rutile needles
with heat treatment. Heat treatment
allows various elements to be
incorporated into crystal. Then
rutile needles form by very gradual
cooling system and induce "asterism".
Inclusions are not present in all
star gems. Some specimens leave
holes or empty areas in the host. Reflection creates from fibres or
fibrous cavity in the crystal.
Types of asterism
There are two types of "asterism",
like Epiasterism and Diasterism.
Epiasterism is creates by the
reflection of light on
parallel-arranged inclusions inside
the crystal. Diasterism is creates
by the transmitting light through
the stone, mostly seen in rose
quartz. To see this effect a lighted
background is essential.
To display the star effect in best
way a gem is cut as cabochon with
the girdle parallel to the needles.
The domed stones are round or oval
shaped.
The special optical phenomenon
"asterism" is most visible in
direct, single pointed light source,
like an incandescent bulb, fibre
optic light, and penlight or in
sunlight.
The distinctiveness of the star and
degree of strait ness of each ray
help to judge the quality and value
of star gems.
More Resources:
Asterism in
gemology
Asterism pictures
Asterism in quartz
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