Sapphire glass is the scratch-proof material used in the new Apple Watch.
Sapphire glass is a synthetic material
Sapphires are most well known as rare, blue-colored gemstones that form naturally. But just like diamonds, sapphire can be made synthetically — in fact, the first synthetic sapphires were made back in 1902.
The process used to make sapphire today is essentially the same as it was a century ago. The natural compound aluminum oxide is ground into a powder, then heated to at least 3,600 °F. For use in a device like a phone, it's then processed into sheets — and without any impurities present, the resulting sapphire glass is a totally clear material.
The iPhone 6 certainly wouldn't be the first use of sapphire glass in a consumer electronics device. The iPhone 5, in fact, makes use of sapphire glass in its camera lens, which makes it virtually unscratchable. Plenty of high-end watches also use sapphire glass faces for the same reason.