Gorilla Glass

Reportedly, fully 20 percent—some 200 million—of the world’s mobile devices incorporate a clear cover made of Corning’s Gorilla Glass brand toughened aluminosilicate glass. Depending on the particular test used to make the determination, Gorilla Glass is seven or eight times stronger than the common soda-lime glasses used, for instance, in most windowpanes.

The exact formulation of Gorilla Glass is a trade secret, but Corning acknowledges that its 1960s-era Chemcor aluminosilicate glass formulation was used as a starting point. Comparing a typical Chemcor formula to that of a typical soda-lime window glass highlights a key difference: Gorilla glass includes much more aluminum oxide than “everyday” glass, and much less calcium oxide.

Gorilla Glass is cast from a hot melt using a special “fusion draw” process, aka the “overflow downdraw method” (Wikipedia), which was also invented by Corning. It improves upon the traditional float glass process (Wikipedia), which is dirtier and less precise than desirable for modern flat-panel display applications.
After Gorilla Glass is cast, it undergoes a critical chemical strengthening process consisting of a potassium nitrate bath at 400°C. Under these conditions, sodium ions in the glass surface exchange with potassium ions in the salt bath. Potassium ions are physically larger than sodium ions, and their introduction into the atomic lattice generates a layer of very strong compressive stresses at the glass surface. This “compressive armor” both resists tensile loads and helps prevent formation of scratches and other small flaws that, as in most glassy materials, are the starting points for major failures.
Corning has just announced the introduction of Gorilla Glass 2, which touts the same performance as the original Gorilla Glass formulation at 20% reduced thicknesses. If you’d like to read more, HowStuffWorks has a good general overview, and Corning’s official literature page is rich with technical detail.