Polyasterism
is the display of more than
one star within a narrow angle
of observation; they aren’t
stars of same star-¬network.
Although quartz and many other
species display polyasterism
the study emphasized the phenomenon
in star sapphire concerning
its high commercial potential.
About 0.1% of Sri Lankan star
sapphire produce and greater
percentage of fashioned stones
display multiple stars or polyasterism.
Relatively common two-stars
are often called "Siamese
twins" and rare three-stars
“triplets” in the
trade. They not only comprise
6-ray stars but rarely 12-ray
stars too, and prevalent among
blue star, star ruby, and all
other sapphire colors, sometimes
fetching higher prices than
that of single star stones.
Apart from usual criteria. symmetry,
of component stars dominates
the valuation of polyasteric
stones. Morphological disturbances
in the single crystal or forming
a compound crystal by bonding/fusing/cementing
several crys¬tals together
by geological reasons or Branching/penetrating
of crystals are the two main
causes that convert a star sap¬phire
into a polyasteric stone. Morphological
disturbances include - basal,
prismatic and rhombohedral -
glides, and are favored in the
same precedence order if one
type is restricted by local
stress. strain or other reason.
Relative displacement of two
or more sections by glide causes
display of asterism independently
in the individual sections arising
from their rutile needle inclusions
- this is the polyasterism in
single crystals. Similarly compound
crystal formed by aggregation
of crystals display the effect.
Figure 7 shows relative displacement
of two sections of kornerupine
crystal by glide, which cuts
the cat's-eye in half and shifts
fire two halves by equal distance.
Basal, prismatic and rhombohedral
- glides takes place similarly
in the sap¬phire stones.
Figures-5, displays clockwise-
polyasterism by basal glide
(0001) [1120], prismatic glide
(1120) [1010], rhombohedral
glide (1011) [1120], and a photograph
of a polyasteric sapphire caused
by basal glide showing two stars.
Figure
5.
Figure
6. Ways of forming a compound
crystal of a polyasteric sapphire.
(a) Three wire frame diagrams
of sapphire crystal branching
at the terminations displayed
along with stones to be fashioned.
(b) Polyasteric stone probably
fashioned from a compound crystal
shows group of rutile cores
at the centre that have revolved
by 30o from the background core
yielding polyasterism. (c) Interpenetration
from a side. (d) Bonding/ fusing/
cementing of two crystals together.
Figure
6.
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