The Apple Watch, due out on Monday, will have a display made from a pretty exotic material: scratch-resistant sapphire glass.

Rumors long held that the iPhone 6 would actually be made of sapphire glass. But those rumors were wrong, and Apple elected to use the material in a watch instead.

Here's an explanation of what sapphire glass actually is — and why it might make more sense for a watch than a phone.

Sapphire glass is a synthetic material

Sapphires are most well known as rare blue gemstones that form naturally. But just like diamonds, sapphires 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 watch, it's then processed into sheets — and without any impurities present, the resulting sapphire glass is a totally clear material.

The Apple Watch certainly isn't the first use of sapphire glass in a consumer electronics device. The iPhone 5 and 5S made use of sapphire glass in their camera lenses, which made them virtually unscratchable. Plenty of high-end watches also use sapphire-glass faces for the same reason.

Why sapphire glass is used in the Apple Watch

The reason why you might want a watch with a sapphire-glass display is pretty straightforward: it's extremely difficult to damage. As materials scientist Neil Alford told Forbes, sapphire is a nine on the Mohs hardness scale. The hardest material on it — diamond — is a 10.

Covering a watch in sapphire glass will make it much more scratch-resistant than the iPhone. This video isn't showing an actual iPhone 6 display, but Alford did tell the Guardian that it seems to be sapphire glass anyway. In theory, the Apple Watch should be just as hard and difficult to scratch as the piece of glass in the video:

To date, smartwatches haven't really taken off. There are a few reasons for this, but one might be that people are hesitant to shell out hundreds of dollars — in addition to buying a smartphone — for a fragile piece of electronics that could get cracked or broken anytime a user merely hits his or her wrist on a doorway. Unlike phones, smartwatches can't be used with a case.

Apple presumably hopes to remedy this situation by billing the watch as a totally scratch-proof technology.

Why sapphire glass still isn't in the iPhone

There are a few possible reasons why Apple still hasn't decided to use the material in its phones.

One is cost. Right now, sapphire glass is more expensive than Gorilla Glass, mainly because it has to be heated to a higher temperature and purified more thoroughly during production. Using tiny pieces of sapphire glass for the camera lens and home button isn't a big deal, but larger sheets covering every iPhone could substantially raise the production cost.

But this price could come down, experts say, if Apple (and other companies) begin using enough sapphire glass to significantly increase the scale of production. In that event, putting small pieces of sapphire glass in watches could be a gateway to putting larger ones in phones.

However, there may be a reason why sapphire glass won't be suitable for phones at all. It's super-hard, but some experts note that it may be a bit less flexible than the Gorilla Glass used in iPhones. This could make it more prone to shattering upon being dropped if a big slab of it were used to support a large, 5.5-inch phone.

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