Some rubies
exhibit a six-rayed star when light
is shined on them, an effect known
as "asterism." The star,
which appears on the surface of a
cabochon-cut ruby, will move when
the gem is rocked. The star is not
fixed on or below the surface of
the stone—as the stone is moved the
rays become thicker or doubled, and
its character changes in a variety
of ways.
The interior of a star ruby contains
many "silk"inclusions.
Needle-like crystals of rutile (titanium
oxide) called "silk" cause
the star to appear, and when light
strikes these rutile inclusions,
which intersect at 60 angles to each
other, a star appears on the surface
of the cabochon. These silk inclusions
also cause star rubies to be semi-transparent.
The star ruby as shown in the photograph
on the next page has a strong red
color and strikes a precarious balance
between high transparency and the
visibility of its star, making it
a miraculous gem-quality stone.
Corundum (the mineral name of ruby
and sapphire) that is red and highly
transparent is polished into faceted
ruby. |
Heavily
included stones that are not expected
to have an attractive appearance
if faceted are generally made into
cabochons, which emphasize their
color. When a cutter comes across
a semi-transparent to translucent
stone containing silk inclusions,
it is fashioned as a star ruby. Star
rubies with a distinct red color
are especially valuable and rare.
In Sri Lanka, time-tested methods
are employed to heat stones to approximately
1,000C (1,850F).
This treatment removes a characteristic
bluish color from
the rubies. This does not cause the
red color to get darker, but when
purplish star rubies are heated they
often become a more pure red. However,
when temperatures exceed 1,300C
(2,370F),
the silk inclusions that cause the
star are altered, causing the star
to disappear. Also, any liquid inclusions
that are present can rupture during
heating and cause the rough to fracture,
making it useless as a gemstone.
Deciding whether or not to heat a
stone requires very specialized knowledge
and experience. |
