Bitcoin prices fell sharply on Wednesday and Thursday after Federal Reserve chief Jerome Powell said he had "serious concerns" about how Facebook's (FB) - Get Report proposed new global cryptocurrency would operate.

Powell said in testimony before Congress on Wednesday that "Libra raises serious concerns regarding privacy, money laundering, consumer protection, financial stability...These are concerns that should be thoroughly and publicly addressed." Powell said a working group had been created to help investigate these issues. 

Facebook announced its plans for Libra in June and the news helped drive the price of bitcoin and other cryptocurrencies higher as Libra is being seen as driving further adoption of bitcoin and blockchain technologies.

The price of bitcoin has fallen almost 8% to $11,793 over the past day, after piercing the $13,000 mark early on Wednesday. Bitcoin prices have more than tripled since the beginning of this year.

Powell's comments on Wednesday came in response to questions from members of the House Financial Services committee, and he made similar remarks on Thursday morning before the Senate Banking Committee.

A number of politicians in the U.S. and abroad have been critical of Facebook's plans for Libra. Maxine Waters, chairwoman of the House Financial Services committee, has said plans to roll out Libra should be halted pending a closer look at the issues it raises. And the Bank of England governor Mark Carney said on Thursday that Libra needed to be "rock-solid" before it could be allowed to launch.

"If you are a systemic payment system, you have to be on all the time. You can't have teething issues, you can't have people losing money out of their wallets," Carney said.

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