Manufactured sapphire—a material that’s used as transparent armor on military vehicles—could become cheap enough to replace the glass display covers on mobile phones. That could mean smartphone screens that don’t crack when you drop them and can’t be scratched with keys, or even by a concrete sidewalk. Sapphire, a crystalline form of aluminum oxide, probably won’t ever be as cheap as Gorilla Glass, the durable material from Corning that’s used to make screens on iPhones and other smartphones. A Gorilla Glass display costs less than $3, while a sapphire display would cost about $30. But that could fall below $20 in a couple of years thanks to increased competition and improving technology, says Eric Virey, an analyst for the market research firm Yole Développement. And since sapphire performs better than glass, that price could make it cheap enough to compete, he says. Sapphire is harder than any other natural material except diamond; by some measures, it’s three times stronger than Gorilla Glass, and it is also about three times more scratch resistant. That’s why Apple uses it now to protect the camera on its iPhone 5. Virey says that all major mobile-phone makers are considering using sapphire to replace glass. “I’m convinced that some will start testing the water and release some high-end smartphones using sapphire in 2013,” he says. An alternative to using pure sapphire is to laminate an ultrathin layer of sapphire with another, cheaper transparent material, maintaining much of the performance advantage of sapphire at a cost comparable to that of the glass typical in mobile-phone displays. For this purpose, GT Advanced Technologies, based in Nashua, New Hampshire, is developing a method for making sapphire sheets thinner than a human hair—much thinner than the nearly millimeter-thick glass used now on mobile phones. (The technology, originally developed for making very thin solar cells, was acquired from Twin Creeks Technologies. See “Startup Aims to Cut the Cost of Solar Cells in Half.”) GT is also cutting the cost of sapphire manufacturing by following the strategy that it used over the last several years to reduce the cost of making crystalline silicon for solar cells.