Sapphire is the birthstone for September – the deep blue precious stone to emerald’s green, ruby’s red and diamond’s scintillating transparency. It also comes in a rainbow of other colors.
Sapphire also has physical properties that are very useful in electronics, optics and engineering. Today we can make synthetic sapphire in a number of different ways.
Sapphire is any gem-quality corundum crystal that isn’t red (red corundum is known as ruby).
It forms 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 the local geochemistry 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 cooling process, when aluminum combines with enough oxygen to balance the ion charges in both elements.
The ionic bond is very strong, and makes corundum very hard, second only to diamond. The crystal is also very stable and 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.
However, when the corundum crystal contains both iron and titanium, a beautiful blue sapphire is born.
If needles of titanium dioxide are present, and they line up in just the right way, a star sapphire is born.
Chromium gives rubies their deep red hue, but just a wee bit of it in a corundum crystal may instead yield the rare and valuable pink-orange padparadscha sapphire.
If 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.
People have been making sapphires for over a century now, notes Daniel Harris in “A Century of Sapphire Crystal Growth,” the paper he presented at the 10th Department of Defense Electromagnetic Windows Symposium in Norfolk, Virginia, in 2004.
Auguste Verneuil, a French chemist, made the first synthetic sapphires in the early 1900s.
In the Verneuil process, powdered aluminum oxide is melted together with whatever metals will give the desired color. Then the mixture is allowed to crystallize.
These flame-fusion crystals appear to be similar to natural sapphires but can be easily recognized as man-made 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, advanced optics and electronics require crystals that more closely resemble natural sapphire.
In 1960, researchers adapted for sapphires the Czochralski “crystal pull” method that had been originally developed to make rubies for lasers.
In this method, aluminum oxide is again used, but this time it’s melted in a nitrogen/oxygen atmosphere. A seed crystal is then dipped into the melt and slowly pulled out, while crystals form 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. Sapphire sheets make 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.
A third way to make sapphire in the lab was developed in the 1970s.
This flux method, described in the sales video below (disclosure: I’m not connected with any business referenced by this article and don’t recommend any product over its competitors), re-creates the high-temperature, high-pressure environment found in nature.
At eight months per crystal, the flux method is way shorter than the millions of years it takes to form a natural sapphire, but it is much longer than the flame-fusion and Czochralski processes, which can grow a crystal in just a few hours to, at most, a couple of days.
The quality is worth the wait.
According to a 1982 Gemological Institute of America paper:
[T]he gemological properties of the Chatham flux-grown synthetic orange and synthetic blue sapphires overlap with the properties of their natural counterparts, with the exception of inclusions. Thus, microscopic examination can provide the definitive means of identification.
Sapphire’s beauty, durability, and rarity make it a precious gemstone. Thanks to the flame-fusion, Czochralski and flux processes, synthetic sapphires are also available for a myriad of uses in optics, electronics and engineering.
A version of this article first appeared online at Helium on 8/15/2011.
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