Trivalent chromium occasionally replaces the aluminum, which is what makes a ruby red.
But this is a rare, rare situation indeed. Rarer even than older societies realized. Many stones that their expert jewelers and gem-setters took to be rubies (aluminum oxide with chromium) were actually red spinels, part of the aluminum group of spinels: A2Al2O4 (A2 representing a magnesium, iron, or zinc cation).
You can't fault them for the confusion, as rubies and red spinels are often found in the very same deposits. Red spinels occur more often, as the rubies will only begin to form after the red spinels have exhausted all the magnesium in that particular location. It's extremely difficult to tell the difference between the two without the sophisticated equipment and methods we have today. As a result, many of the world's most famous imperial decorations include not rubies, but red spinels.
The most famous of these is perhaps the Black Prince's Ruby, in the Crown Jewels of England.
The link has the entire dramatic and bloody life of the stone, but in a nutshell: it was first mined in what is now Tajikistan, in the great ruby and spinel mine of Kuh-i-Lal.
The ruby made its way westward and eventually surfaced in Spain, where it changed hands a few times. Eventually it went to Edward of Woodstock, also known as "The Black Prince," and with him it went to England.
The ruby had a few narrow scrapes in England: it was almost lost in a military campaign against the French; it was almost junked (along with the rest of the original crown jewels of England) by Oliver Cromwell; it was nearly stolen by Thomas Blood; it was nearly consumed by fire in 1841, and was a point of concern during Hitler's blitzkrieg bombing of London.
Nonetheless, it survives as a brilliant and beautiful specimen, not of a ruby, but of a red spinel. Today, spinels are easily synthesized in labs. Because of the variety of color with which one can infuse them, spinels are often used as an imitation gemstone: sapphires, emeralds, alexandrites, and others, in addition to rubies. It might therefore be tempting to turn your nose up at a red spinel as a second-rate imitation, but remember this: those spinels were good enough for English, Russian, and Persian royalty—are you so snobby as to be above royalty?
This raises an interesting ontological point, then: is a ruby simply and only the aluminum oxide specimen with chromium? When many of what we consider to be the preeminent specimens of "rubies" technically aren't? What's more important, the chemical definition or the cultural shorthand marker? Remember, the most distinguished jeweler in field the 18th century would consider your lab-grown red spinel a fine and outstanding ruby.
More About Rubies:
Rubies on MinDat.org
Rubies on Flickr
More About (Red) Spinels:
Spinels on Minerals.net
Red spinels on Flickr
Spinels
Spinels Buyer's Guide
