Geology Home Page physical geology historical geology planetary gems
Roger Weller, geology instructor
by Katie Stilwell
Star Rubies and Sapphires
Corundum is a mineral commonly used in gemstones; known as a ruby if it is red
or a sapphire if it is blue. The type of corundum uncommonly used is in the form
of a cabochon and is referred to as a “star ruby” if it is red, or a “star
sapphire” if it is blue. The star in the corundum is called an Asterism,
and it caused by a ‘silk’ in the crystal called “rutile”. Rutile causes a
six pointed star to show when light hits the surface of the sapphire. Just as in
other gemstones, the quality of the corundum is reliant on cut, color and
clarity. However one must not be hasty to purchase a gem such as a star
sapphire, as many of the gems in the market today are synthetic or heat treated.
With a gem such as the star sapphire or the star ruby, often it is evident that
the jewel has been tampered with to achieve a certain shade or color.
The cut is essential to the quality of the gem because if cut too much or not
enough the asterism in the sapphire will either not be bright enough or the six
pointed star will almost fade altogether. Since the cut also helps depend on the
size of the gem, cutting is part of the overall quality; an incorrectly cut
cabochon will dramatically decrease the size, quality, and beauty of the jewel.
The worth of a star sapphire also greatly depends upon the color, as in almost
all other gemstones; the deeper the color, the more expensive the corundum. A
deep blue star sapphire with a clear six pointed star is very hard to come by. Corundum itself is an aluminum oxide, which means that any time aluminum ore is
exposed to oxygen; it oxidizes itself and converts into the many forms of
corundum. The general issue is either the star in the deep colored crystal is
hazy and not well pronounced, or the well pronounced star is inside a lightly
colored gemstone. In star sapphires, the majority of the gems are light blue or
a grayish blue color; in star rubies the high quality gems are a dark purple
(Photo courtesy of Roger Weller)
As you can see, the color of the ruby, which is the red variety of corundum, is
a dark purplish red; most of the darker colored, higher quality star rubies on
the market come from Burma, India, and Sri Lanka. Also, the star is a bit hazy
and not as sharp as an extremely expensive star ruby might be, however, this
particular specimen still has a great star for its color. These Sri Lanka and
Burmese star rubies generally go for a high price on the gemstone market.
Natural star rubies are one of the few types that are easily discernable from
synthetic or heat treated gems. Heat treatment is where a gemstone is placed in
a high temperature zone, much like an oven or furnace type of kiln, and is
heated to change the color of the stone. These types are somewhat hard to spot
in faceted gemstones sometimes, but in this type of market the rubies are more
of a pink color instead of a red. They may be passed as a star ruby, but instead
they are more of a pink variety of corundum sold as a sapphire that has been
(Photo courtesy of Simply Sapphires, New York).
The difference is clear between the natural and the synthetic star rubies. This
has a well pronounced star, but has a bubblegum pink color to it instead of the
wine color of the Burmese gem. This is the effect of the heat treatment; the
color is altered from its original state in the attempt to enhance the color,
and hopefully the price, of the gemstone.
The color of the rest of the ‘star’ varieties vary greatly, from pink, blue, or even black. Generally, the sapphires sold on the market consist of a light blue or grey blue variety, because of the quality of the stars, the color is sacrificed for the overall clarity of the asterism.
(Photo courtesy of Simply Sapphires, New York).
The light blue or grey colored star sapphires are the commonly sold star
sapphires on the market. This particular specimen has a distinct asterism, is
lightly colored and has streaks of darker blue enclosed in the cabochon. The
higher quality gems often are a very deep color, whatever color that may be. Often, the most expensive star sapphires are the deep blue colored specimens,
regardless of the quality of the star shaped asterism in the middle. Personally,
I enjoy the deep blue colored ones, but I am also fond of the black stones as
well. The black stones are a very distinct, bold color to them, and look
extremely attractive in both white and yellow gold. The blue variety of star
sapphire is more popular than the black, since the common thoughts are that
sapphires are blue. These stones are often used in jewelry, such as rings and
pendants. The more vintage-style rings are very popular if they can be found. In
my opinion, the vintage rings are the most attractive with this gemstone,
especially if they are genuinely vintage, such as this ring.
(Photo courtesy of Katie Stilwell).
The pronounced star, deep blue color, enclosed in a vintage-style gold setting
makes this beautiful ring a high quality specimen. The dark blue color is quite
rare and when paired with the distinct but not too sharp star, if the ring were
to be placed on the market it would fetch quite a hefty price. Of course, when
shopping for any type of gem, one must be also be aware of the last important
aspect of the gemstone: clarity.
Clarity of a star ruby or star sapphire is not as apparent as in a
transparent gemstone, when the flaw is sometimes extremely clear; however they
are not without their flaws. The translucent nature of the star corundum
varieties are easiest to judge by their asterisms, the six pointed star that
appears when it is hit by light. The asterism is caused by the mineral Rutile,
which is a titanium ore that has been oxidized. Inclusions of the material
Rutile into the corundum cause the asterism, and sometimes give the gem a cloudy
appearance. The appearances of the cabochons depend heavily on Rutile, which is
responsible for the asterism in the star sapphires and rubies, and is also the
cause of the cat’s eye and tiger’s eye in those gemstones. Rutile is commonly
seen in the aluminum and silicon varieties of gemstones; such as corundum and silmanite. These inclusions of Rutile into the corundum make the gem extremely
valuable, if cut and polished correctly, and if the asterism and color are of a
The asterism itself is the main focal point of the star corundum,
the muted stars are not too valuable, but the sharp stars are not either. The
best asterism is not too hazy, not too sharp. Though there are people who prefer
them one or the other. The stones with sharper stars often sell well, as they
are clearly different among the many faceted gems.
(Photo courtesy of Roger Weller).
Stones that are lacking a well defined star, are cloudy in appearance, have
markings, striations, or other flaws in them, and are not considered a good
specimen for the jewelry market. Cabochons that have striations or grooves in
them will considerably drop the price, and even with a deep color to them, gems
with a weak star or simply a cloudy appearance are also not considered as
valuable. Some consider the striations, or the appearance of the actual crystal
as quite lovely, and in that case, they can be bought quite cheap; however the
majority of consumers enjoy a clear gem as opposed to a cloudy one.
(Photo courtesy of Roger Weller)
For example, these star rubies are generally not too desired; the asterism is
barely noticeable, and there are grooves on the surface of the cabochon;
however, there is a deep color to them which makes them still beautiful, but not
so valuable to many. Though the asterism assists in the overall clarity of the
gem, but the clarity is mostly dependent on the translucency of the gemstone. In
this instance the gem does not have much clarity, but is consistent with its
rich color and mostly well cut appearance. Though it may not have the clarity of
a flawless ruby, it is still unique in its own aspects.
Such as in all gemstones, the star ruby and star sapphire’s quality depend greatly on the cut, color, and clarity of the gem to be considered valuable, but the many varieties of corundum make it easy to decipher your favorite easily; whether it be a translucent or transparent, each is beautiful and unique in its own way. That is one of the things I have learned in our Geology class; among the other things I have learned is to always look down, you never know what you may find, how to shop for a house, what to look for and what to avoid, and that there are many, many ways to die a horrible and gruesome death. At the end of the day, I do not think I will ever visit California for more than a day, and I will probably never set foot in Yellowstone National Park, but I may have some of the finest jewelry you have ever seen for the lowest prices you can find; and I’d like to thank Mr. Weller for that.