Sapphire

Sapphire is crystalline aluminum oxide, the same material as the gem. Natural blue sapphires get their color from traces of iron and titanium; industrial sapphire is pure and colorless. It's extremely hard, scratched only by a few materials such as diamond, and it stays stable at high temperatures and in harsh chemicals.

It's grown as a single crystal, with its atoms lined up in one unbroken pattern through the whole piece. In the Kyropoulos method, the most common for LED sapphire, high-purity aluminum oxide is melted in a crucible at over 2,000 °C. A small seed crystal touches the surface, and the melt is cooled very slowly, over days to weeks, so the crystal grows into a large, clear boule that can weigh more than 100 kilograms. The boule is cored, sliced into thin wafers and polished to a mirror finish.

Gallium nitride isn't easily made as large wafers of its own, so blue LED chips are grown on sapphire. Its crystal pattern is close enough for the layers to grow well, it survives the hot growth process, and it's transparent, so light can escape through it. It also covers many watch faces.