Citi downgrades this high-flying Apple supplier because iPhone upside is priced in

A still image from a Corning Gorilla Glass 4 promo video
Source: Corning

Citi Research lowered its rating on Corning to neutral from buy, saying the glass and specialty materials maker is fully valued after a significant increase in its share price.

The company's Gorilla glass product is used in Apple iPhone displays. Corning shares are up 32 percent in the last 12 months through Thursday.

"Investors are keenly focused on the iPhone 8 'supercycle' as it pertains to Corning but we believe this is well reflected in estimates," analyst Stanley Kovler wrote in a note to clients Friday.

The analyst also warned investors on the potential for increased display product factory capacity, difficult smartphone sales comparables and lower profitability from its optical business late next year.