Sapphire is the birthstone for September and the blue precious stone to emerald’s green, ruby’s red and diamond’s scintillating transparency. This gem variety of the mineral corundum also comes in the rainbow of other colors found in fancy sapphires.
However, sapphire is more than just a pretty face. Its optical properties, chemical stability and inertness, as well as physical qualities of extreme hardness and thermal resistance, have made it almost as valuable in engineering, electronics and optics applications as it is in fine jewelry. Today people have learned how sapphire is formed and can make synthetic sapphire in a number of different ways.
♦ Natural sapphires
Sapphires form when molten rock deep underground starts to cool down after an episode of volcanism or tectonic deformation (metamorphism). This happens very slowly, over millions of years, since the magma body is well insulated by the surrounding rock and stays at an extremely high temperature for a long time. It’s also under tremendous pressure. Crystals form inside the magma as dissolved elements interact with each other and the outside rocks and start to come out of solution. In outer zones where a little molten rock has pushed into cracks in the native rock and heated up any water that was there, a similar crystallization is underway as the circulating hydrothermal fluids cool.
Corundum is a simple oxide and usually starts to crystallize pretty early in the process, when aluminum combines with enough oxygen to balance the ion charges in both elements. This ionic bond is very strong, and makes this crystal very hard, second only to diamond on the Mohs scale, and about four times as dense as water. Corundum is also very stable, and the mineral is unaffected by acids or high temperatures.
The colors of sapphire happen because its structure allows substitution of different metal ions for the aluminum. Pure corundum is colorless, and if some titanium gets into it through a charge transfer process, it will still have no color. A star sapphire is made when the crystal contains needles of titanium dioxide that are aligned just right for the optical effect of asterism (the six-pointed star).
A touch of chromium (which also gives those sapphires people prefer to call rubies their deep red hue) can yield the rare and valuable pink-orange padparadscha sapphire. When chromium and iron are both present, purple and mauve sapphires form (vanadium can also cause a purple color, but it is very rare in natural sapphires). Iron by itself will turn the corundum light yellow or one of the other fancy sapphire colors. When the crystal contains both iron and titanium, the beautiful blue sapphire is born.
Natural sapphires are found as three-sided crystals (trigonal), colored sections of a larger corundum crystal, or as pebbles washed down into a stream bed after the softer rock formation where they once were has weathered away.
The deep, velvety blue that usually comes to mind when we think of sapphires is found in stones from Kashmir, hence, the name Kashmir blue for gems that may or may not come from that area but do have the right color. There is a tremendous variability in sapphire colors throughout the world.
Myanmar produces sapphires, but those of Thailand are world famous, although their high iron content makes them darker than the ideal blue. Blue, green and yellow sapphires have been found in Vietnam since 1991, and many other colors of sapphire are found there, too. Australian sapphires are generally dark, but green sapphires from Down Under are said to be the best of their color. In the USA, bright blue sapphires come from the Yogo, Montana, area. Lilac and purple stones are also found. In Africa, Kenya has a variety of dark colored sapphires, while Tanzania’s sapphires come in a wide range of pastel colors, as well as green and blue.
♦ Synthetic sapphires
“We have been making sapphires for over a century now”, notes Daniel Harris in “A Century of Sapphire Crystal Growth,” (PDF file) the paper he presented at the 10th Department of Defense Electromagnetic Windows Symposium in Norfolk, Virginia, in 2004.
It’s significant that even the defense industry has uses for this versatile gemstone. Sapphire is now affordable enough to be used industrially because manufacturers have learned how to make it.
August Verneuil, a French chemist, made the first synthetic sapphires in the early 1900s. In the Verneuil process, powdered aluminum oxide and whatever metals that will give the desired color are melted and then allowed to crystallize. These flame-fusion crystals are very similar to natural sapphires but can easily be recognized by a trained gemologist.
The Verneuil process is still used today with only minor modifications because it produces low-cost sapphires of good enough quality for many applications in jewel bearings and precision instruments. However, in the mid-20th century, advances in optics and electronics required sapphire crystals of a higher quality.
In 1960, researchers adapted for sapphires the Czochralski “crystal pull” method that had been originally developed to make rubies for lasers. In this method, alumina is melted in a crucible in a nitrogen/oxygen atmosphere. A seed crystal is dipped into the melt and then slowly pulled out as aluminum oxide from the melt crystallizes on its surface.
Today, sapphire crystals are grown through a number of different adaptations of these two processes, depending on how the final crystal will be used. Optical filaments of sapphire can be made that resist high temperatures, or sapphire sheets for scratch-proof windows on such things as bar coders and wrist watches, as well as wafers for sapphire-on-silicon chips for the electronics industry, and optical domes to protect aircraft instruments.
There is a third way to make sapphire in the lab. In the Chatham flux method, as Tom Chatham of Chatham-Created Gems explains in an informal and very informative sales video on YouTube, the high-temperature, high-pressure environment found in Nature is re-created, and the sapphire crystals are allowed to form naturally. Gemologists agree that Chatham gems are very difficult to distinguish from naturally formed jewels. At eight months per gem, the Chatham flux method is much shorter than the millions of years it takes to form a sapphire naturally, but it is much longer than the flame-fusion and Czochralski processes, which can grow a crystal in just a few hours to a couple of days at most.
Natural sapphires form deep underground when aluminum oxide crystallizes into the mineral corundum under extremely high temperatures and pressures and then gains color from iron, titanium or other metal impurities. Their beauty, durability and rarity have made these gems precious down through the ages. Today, sapphire’s useful physical and chemical properties, as well as its desirability as a gemstone, have led to the production of synthetic sapphires either by chemical methods, as in the Verneuil and Czochralski processes, or by duplicating Nature’s extreme conditions with the Chatham flux method.