Apple

Sapphire bound for future Apple products has begun shipping from a supplier's Arizona-based factory, according to UBS Securities.

"Checks find [GT Advanced Technologies'] Arizona factory likely [started] producing sapphire last month. Our own checks found that for the first time GT shipped some small quantities of sapphire made in its Arizona fab to one of Apple's partners in China who is in charge of making sapphire covers," UBS Securities' Stephen Chin wrote in a research note released Wednesday, as reported by 9to5Mac.

Apple signed a deal with GT Advanced Technologies back in November. That deal stipulated that Apple provide GT with $578 million in prepayments "at an Apple facility in Arizona." GT owns and operates the furnaces that produce the sapphire.

So, the question becomes, what will Apple use the sapphire for? Apple CEO Tim Cook said in February that the sapphire production facility was for a "secret project."

Speculation ranges from a future Apple smartwatch, aka iWatch, to a high-end . Regarding the latter, Ming-Chi Kuo, an analyst at KGI Securities, indicated earlier this month that a 5.5-inch Apple "phablet" may get a sapphire screen cover.

Sapphire is a hard, scratch-resistant material currently used on the iPhone 5S' home button. In theory, it could replace the Gorilla glass that Apple currently uses for its iPhone.

GT Advanced Technologies declined to comment.