NEW YORK (TheStreet) –– GT Advanced Technologies (GTAT) shares sank 5.5% to $15.17 following a negative analyst report.

CLSA analyst Mark Heller halved his estimates of GT’s sapphire capacity and revenue for 2014. He cited lower than expected growth in GT’s shipments of its sapphire screens for Apple (AAPL) products, indicating that “GT is having issues ramping the new furnaces, which adopt leading edge technology to grow larger boules for mainstream supplier.” He also noted that GT is not supplying sapphire for the upcoming iWatch, despite consensus expectations that it would.

However, Heller’s expectations for the company are positive in the longer term, noting he "expect[s] GT to solve its yield issues over time and see no change to the view that Apple will increasingly adopt sapphire cover technology over the next few years.”

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Last week, UBS downgraded GT stock to “neutral” from “buy” and cut EPS to 12 cents from 17 centa share, but maintained a price target of $22. UBS analyst Stephen Chin wrote that the company will struggle to reach the high end of its guidance: “We believe GT can still hit the midpoint of its sales guidance of $600-$800M but believe our checks of a slower ramp in 2Q14 makes the high end more challenging.” The company’s sapphire ramps were slower than expected and, Chin wrote, “We are not sure why the ramp slowed.” Five days before Chin’s report, GT posted an all-time high of $20.54. Since January, GT shares have nearly doubled.

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