Bitcoin has one notable new detractor. 

Federal Reserve Chair Janet Yellen said the cryptocurrency is not only speculative, but also is not a stable store of value and not a legal tender. 

"Bitcoin at this time plays a very small role in the payment system," Yellen said. "It is not a stable source of value and it does not constitute legal tender. It is a highly speculative asset."

Yellen spoke on the digital currency, which has rallied about $5,000 in the past week, following the announcement that the Fed hiked interest rates for the third time this year at its meeting ended Wednesday, Dec. 13.

Yellen has commented very little on bitcoin during her tenure, last offering her stance on the asset in 2014. At that time, she said the Fed wasn't in an authoritative position to regulate bitcoin.

Bitcoin had already been falling Wednesday afternoon, down 3.9% on the day to $16,515.68. 

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