Having a more dependable supply could allow Apple to innovate with the glass.
Second in hardness to diamond, sapphire glass is durable and scratch-resistant. It is already used to cover the iPhone 5 camera lens and home button on the iPhone 5s.
The glass is also a luxury item. The nearly unbreakable sapphire glass can cost up to seven times as much as Corning's Gorilla Glass, which is used to make most smartphone screens. In fact, only the $3,000-plus Vertu smartphone uses it for its touchscreen.
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Because of the cost burden, most analysts agree that a large iPhone or any other large-format smartphone with a sapphire screen would be too expensive for the current market. But if Apple needed a smaller screen—perhaps for a future iWatch—it would have broader use for sapphire glass, analysts said.
Some high-end watchmakers, such as Tourneau and Tissot, already use sapphire as a protective cover.
But a potential Apple smartwatch isn't the only reason the tech giant is getting its hands on more glass, according to industry experts.
Potential for sapphire glass in mass production of small home buttons and camera lens covers for Apple devices may have also spurred the partnership, said Richard Son, senior analyst at industry research group IHS.
Sapphire's main use currently is in LED lights. But Taiwan-based analyst firm LEDinside said the demand for non-LED uses will increase at least 20 percent next year as more phonemakers follow Apple's lead.
LG's latest G2 phone and ZTE's newly released Nubia 5 feature sapphire camera lens covers.
With wider interest for sapphire glass in smart devices and all GTAT's production of it going to Apple, LEDinside predicts that the present oversupply will become a shortage by 2015, which will drive up prices. The following chart, based on data from the firm, shows the demand to supply ratio of sapphire substrate (or glass).