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Star rubies and sapphires
Corundum often contains very fine needles of
rutile (Ti07) arranged in intersecting lines in accordance
with the symmetry of the crystal. These apparently develop
when the stone is formed: as the temperature falls, the TiOz
is no longer soluble in the AI203 and forms separate crystals.
When the needles inside the corundum are particularly numerous,
and the stone is cut en cabochon, with its base (or widest
diameter) parallel to the base of the prism, a silk-type reflection
is visible in bright light; it is fairly mobile and has the
appearance of a six-rayed star, the closer and thicker the
rutile needles inside, the more clearly this stands out. When
reasonably pronounced, this effect is considered attractive,
contributing in the past to the aura of mystery surrounding
some gemstones.
Appearance The most striking
phenomenon of rubies and sapphires is the development of the
six-ray star, arranged in perfect symmetry, which shifts its
center as the stone is moved. It is clearly visible under
a single light source such as the sun or a lamp; much less
so in diffuse light. If two or more powerful light sources
are set close together, as many stars (their centers not far
apart) can be seen in the stone. Each light produces its own
star, which is basically a reflection. The effect is usually
less pronounced in more transparent stones. The ground color
can be ruby red (or an almost grayish, dull red), in which
case it is known as a star ruby, or sapphire blue, in which
case it is known as a star sapphire, but it can also be blue-gray
smoke gray, or blackish, all of which come under the name
of asteria, a generic term also applied to ruby and sapphire.
Such stones are invariably given a round or oval cabochon
cut.
The most highly prized are the ruby-colorec
(provided they are not the opaque, grayish red of some Indian
rubies) and sapphire blue varieties. The others are less valuable,
but still sought after, provided the star is clearly visible
and they are not too small (3 or 4 carats, at least). Some
star stones may weigh 10 carats or more. Distinctive features
The star with its distinctive mobilih is characteristic, having
six rays, unlike star diopside, for example, which has four.
But to be certain of distinguishinc star corundum from the
widely divergent (but very few; other gems which can display
the phenomenon of asterism, one normally has to measure the
physical properties.
Occurrence Rare but magnificent
star rubies and sapphires are found in Burma, although most
star corundums come from Sri Lanka, usually being light blue
or gray. Dark asteriae are found in Australia, and dull red,
opaque specimens are found in India. Despite being rubies
and displaying the phenomenon of asterism, these stones are
not very attractive.
Value Star rubies and sapphires
of good or even above average color are distinctly valuable,
as much so as faceted stones of similar color. The value of
the grayish or dark asteriae is lower, though not much, for
unusually fine specimens. On the other hand, comparatively
small stone~ of insipid color or with a poorly defined star
are worth a great deal less, and the same applies to dull-colored
rubies, which often have a broad, smudgy star.
Simulants and synthetics Because
of their undoubted attraction, star stones have been imitated
in various ways. Efforts have been made to produce a star
by engraving it on the base of a cabochon, or lining the base
with a sheet of metal engraved with a six-rayed star. Milky
quartz, which exhibits a weak form of the same type of asterism,
has also been used, the base of the cabochon being covered
with blue lacquer to give color to the stone and increase
the contrast with the star; but the effect is somewhat different
from that of natural star stones. In the last few decades,
however, some manufacturers of synthetic corundum (using the
Verneuil method) have found a way of producing both red and
blue star stones with very pronounced stars (more pronounced
than the natural versions), which are not as a rule too transparent
and have an attractive, lively color; and these have been
a great success in the United States.
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