Note: A version of this article appeared at Helium on August 4, 2011.
The loose gemstone that Alfred the butler described to Master Bruce in The Dark Knight as “a ruby the size of a tangerine” would have been much less interesting had he been talking about “a lump of aluminum oxide with some chromium in it.”
Ruby, the gem
Rubies and sapphires are basically the same thing – corundum. The term “ruby” is given only to corundum that is a special shade of red.
That Burmese bandit was insane: completely flawless natural rubies, of any size, are very rare and worth millions of dollars.
However, many rubies with inclusions are still high-quality gems. If a ruby has just a few inclusions or its color is a little off, it can be heated almost to its melting point, causing the aluminum oxide to re-form. This eliminates gas and fluid inclusions and also makes chromium combine with different atoms, improving the gem’s red color.
Heat treatment is common for rubies and accepted in the gem world, as long as it is disclosed to the buyer.
Rubies with more inclusions can be carved (using diamond tools) and sold as jewelry. In fact, ruby sculptures are often beautiful works of art in their own right.
Some rubies have a milky appearance that comes from needle-shaped inclusions of titanium oxide crystals called rutile. The value of rubies with inclusions soars again when their rutile needles are aligned just the right way to produce the optical property of asterism: a star ruby.
One of the largest star rubies known is the 138.7-carat Rosser-Reeves ruby, in the Smithsonian.
To tell rubies and garnets apart, check the hardness. Natural rubies have a Mohs hardness of 9 (which makes our little mouse friend up there all the greater an achievement!), while garnet is only 6.5 to 7.
Other, more famous stones have been mistaken for rubies, too.
After the 18th century, when modern science discovered the chemical and physical structure of ruby, people realized that some of the most famous red stones in a number of crown jewel collections, including the Black Prince’s Ruby and the Timur Ruby in the British Crown Jewels, were actually spinels.
Garnet is a silicate mineral, but spinel is a magnesium aluminum oxide mineral that more closely resembles ruby. It’s also harder than garnet (though not quite as hard as ruby).
Even setting aside the topic of artificial gemstones for another post, there are rubies and then there are “rubies” in the natural world (and the marketplace). To tell these apart, it does help to know a little bit about what’s going on beneath the surface of these pretty objects.
Ruby, the mineral
Perhaps you already knew that rubies and sapphires are the same mineral – corundum – with different colors, but did you know that corundum is the crystalline form of the same oxide we use in industry to make aluminum metal products?
Aluminum is an earth metal, one of the boron group on the periodic table. Along with boron, gallium, indium, thallium and ununtrium, aluminum ions are trivalent, that is, they have three electrons in the outer energy layer. This is an unstable configuration and makes aluminum too reactive to stand alone, though as the most abundant metal in the Earth’s crust, it is found in over 200 different minerals.
In corundum, aluminum is bound with oxygen to make aluminum oxide crystals, with the chemical formula Al2O3 showing that two atoms of aluminum share electrons with three atoms of oxygen. While aluminum and oxygen aren’t very dense by themselves, their covalent bonds are very strong and short. These bonds pull the atoms close together, making corundum about four times as dense as water. That’s also the reason why it is the second hardest mineral in nature, with a Mohs scale hardness of 9, compared to diamond’s hardness of 10.
Pure corundum is rare in nature. Sometimes chromium gets into it. This member of the transition metals isn’t nearly as common as aluminum in the planet’s crust, but it does have the same sort of trivalent ions and so can go into solution with corundum, with Cr3+ (chromium ions) taking the place of some of the Al3+ (aluminum ions).
The aluminum doesn’t absorb light, which is why corundum itself is colorless, but chromium does absorb some wavelengths of light, giving the (Al, Cr)2O3, or ruby, both its red color and the physical property of fluorescence that enhances the gem’s beauty as well as making it usable in a laser.
Okay, a little chemistry goes a long way.
Rubies are beautiful, though, and very useful. When you get right down it, these “red sapphires” really are in a class all their own.
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