The risk is that Apple's plans for GT Advanced Technologies' sapphire is more limited, resulting in sales missing hyperbolic expectations. Due to its immensely speculative nature, potential investors in GT Advanced Technologies should keep their positions small -- no more than 1%-5% of a well-diversified portfolio.
it's already had a huge jump in price. It's still a speculative stock since neither apple or gtat can officially confirm their business relationship or apples intent on using gtat for apple products. There is as much risk for the downside as upside.
Lastly, apple has already probably made money off of their investment even without using gtat sapphire.
Lastly, rumors of a delay in apple production is NOT good for GTAT.