what are star rubies?
Star
Rubies are a rare variety of the gemstone Ruby. These magnificent gems display
a sharp six-rayed star which seems to glide magically across the surface of the
gem when the latter is moved. This is caused due to an optical phenomenon known
as "Asterism". The star is best visible when the star ruby is seen in
a single light source such as sunlight and spotlight.
what causes the star-effect in star
rubies?
Star-effect
or "asterism" is caused by the dense inclusions of tiny fibers of
rutile (also known as "silk"). The stars are caused by the light
reflecting from needle-like inclusions of rutile aligned perpendicular to the
rays of the star. However, since rutile is always present in star rubies, they
are never completely transparent. In fact, star ruby is one of those few
gemstones which actually require inclusions (i.e. rutile).
are star rubies rare?
Today,
out of every 100 rough corundum (ruby and sapphire) mined; only three will have
stars apparent after being cut into cabochons. Out of these three, one will
have a poor color but a good star; one will have a poor star but a good color
and only one out of hundred will have both a good star and a good color! Most
original sources of star rubies such as Mogok (Burma) have run out of supplies.
Worldwide production today is yielding a very small quantity of fine stars
(especially in sizes above 5 carats). Star rubies are becoming rare as most
rough today is cut into faceted stones after heating instead of being cut into
cabochons to display the star. Till the 1960’s star rubies fetched more prices
than faceted rubies!
how to determine the value of a star
ruby?
The
value of Star rubies is basically influenced by two things:
(a) The strength and sharpness of the star,
(b) Intensity and attractiveness of body colour
cutting
Cutting
is very important to exhibit the star effect in ruby. They need to be cut into
medium to high, domed cabochons to get the perfect star effect. The cabochons
must be round or oval-shaped. The stone should not be too flat on the top or
too heavy on the bottom. Many a times, bottoms of star rubies are kept
unpolished so that the light is trapped inside the gemstone.
(Left: A Rough Star Ruby)
colour
Generally
rubies are red. But star ruby is usually not red. Most star rubies are
available in pinkish-red, purplish-red or purplish-brown colour. The
distribution of colour is often uneven, in stripes and spots. A red ruby with a
perfect star is very rare and costly. As a general rule, pink star rubies have
the best stars.
clarity
As mentioned earlier, due to the presence of rutile (silk) star
rubies are never transparent. Most natural star rubies found are translucent
and opaque (mostly opaque).
carat size (weight)
Normally,
star rubies are available in the sizes ranging from 1 carat to 15 carats. Fine
star rubies of large sizes are extremely rare and cost a lot of money.
star sharpness
The
star should be sharp (not blurry) and silvery / milky white. The best way to
test the sharpness of the star is to look at the star ruby in sunlight with the
dome facing the sun. All six prongs should be straight and equally prominent.
The complete star should be centered in the middle of the gem, but should also
be able to glide effortlessly around the cabochon.
sources of star ruby
Most
rough rubies that can be cut into stars are mined from Burma (Myanmar),
Srilanka, India, Africa, Vietnam, Thailand, Afghanistan, Pakistan, Australia,
Cambodia, United States. The most commonly available Star Rubies are usually
from Africa and Indian.