Put simply, Chinese tourists will have less buying power at Tiffany's stores as their yuan holdings are converted into fewer U.S. dollars.
The Chinese yuan has fallen more than 5 percent against the dollar this year as trade relations between the U.S. and China intensify. The U.S. has slapped tariffs on billions of dollars worth of Chinese goods, to which China has retaliated in kind. The yuan is also trading around its lowest levels in a decade against the dollar.
There is speculation China's government is allowing the currency to weaken as a way to boost their exports and fight back against America.
Shares of Tiffany fell 0.9 percent in premarket trading. The company's stock has risen over 31 percent this year as of Thursday's close at $137.10 per share.
Oppenheimer reiterated its price target of $145 per share, despite the downgrade in rating. The firm says it looks "very favorably upon longer-term prospects for" Tiffany but the possibility of the U.S. trade war with China claiming its next victim serves as a large near-term risk to the stock.
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