At Apple’s latest product unveiling tomorrow, CEO Tim Cook may sing the praises of super-hard sapphire, and proclaim it as the perfect screen material for its new iGadgets.

Cracked screens are a common problem for smartphone users, so virtually unscratchable, unbreakable screens could make for a compelling marketing campaign. But just how durable sapphire will be is debatable. Much will depend on how well Apple can manufacture it in large quantities, and how it incorporates the material into its devices.

Rumors have been circulating for months that Apple plans to use sapphire, a substance that is significantly harder than glass, in its latest iPhones, and perhaps its first iWatch. The company’s decision to invest about $700 million in an industrial sapphire plant in Arizona starting last year has added considerable weight to the theory.

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