Why are rubies red?

November 1, 2013
This article appeared in Berkeley Engineer magazine, Fall 2013

Ron Gronsky (M.S.’74, Ph.D.’77 MSE), professor of materials science and engineering, is rated by Princeton Review as one of 300 best professors nationwide. “He makes the material interesting by interjecting great stories and examples of real-life applications,” posted one student on the website Rate My Professors. Here Gronsky describes one favorite example from his E45 course, “Properties of Materials”:

“If you are ever in a jewelry store perusing the ruby collection, wait until a salesperson is within earshot and say, ‘These are some really nice sapphires.’ That should capture attention, most likely causing the salesperson to say, ‘No, no, no…those aren’t sapphires, they are rubies!’ Having taken the bait, your salesperson can next be informed, maybe with slightly more volume, ‘But rubies are nothing more than impure sapphires.’ You might now have a few salespeople nearby with their dander up. Wait until all are calm enough to pay attention, then give them the lecture they’ve earned:

Rubies are a form of aluminum oxide, or alumina, in the parlance of the ceramics community. The most common form of alumina is best known by the mineral name corundum, which in powdered form is a common abrasive used in grinding wheels or papers. Alumina is also used in many commercial products, such as coffee cups, dinnerware, sink and bathtub porcelain, even Portland cement.

Some geological conditions generate a rare version of alumina, perfect single crystals, a family of gemstones known as sapphires. The purest sapphire with low residual porosity is perfectly transparent. However, nature frequently injects a number of impurities, small amounts of metals other than aluminum that cause the sapphire to assume many different colors. Blue is the best known, but sapphires can also be green, yellow, brown, pink, purple or smoky black. The color comes from the effect of the impurities on the electronic structure of the alumina, modifying its ability to absorb and re-emit light of different wavelengths.

A particularly special case occurs when the metallic impurity is chromium, in concentrations from 0.001 to 1 percent, resulting in a brilliant red color and the most expensive of all gemstones: rubies.

At this point, if your sales staff members are still listening, you have probably rendered them vulnerable enough to negotiate price, wary that they are now speaking to an expert!”