Bitcoin fell on Monday, interrupting its impressive recovery after Elon Musk said prices “look high”.

The world’s most valuable cryptocurrency fell more than 10% to a price of $ 51,993 on Monday morning, according to data from Coin Metrics. At one point, bitcoin fell below the $ 50,000 level, dropping to $ 47,700.

It was not immediately clear what was behind the bitcoin shift on Monday. On Saturday, Tesla’s CEO said prices for bitcoin and rival ether token seemed excessive. Bitcoin rose to more than $ 58,000 on Sunday, but has since changed course.

As of 10:25 ET, bitcoin was trading around 8% cheaper at a price of $ 53,375. It is important to note that price fluctuations of more than 10% are not uncommon in cryptography. Bitcoin has already risen to almost $ 20,000 in 2017, before losing 80% of its value the following year.

U.S. Treasury Secretary Janet Yellen warned on Monday that bitcoin was a “highly speculative asset” and said she is concerned about investors’ loss of money.

“It is an extremely inefficient way of conducting transactions, and the amount of energy consumed in processing these transactions is impressive,” said the former Federal Reserve chairman.

The digital currency is still up more than 80% this year. Last week, bitcoin reached $ 1 trillion in market value for the first time – it is now back below that mark, according to CoinDesk. The token gained momentum with news from major Wall Street banks and Fortune 500 companies worrying about cryptocurrencies.

Musk recently declared himself a bitcoin fan, calling it a “good thing” and saying he thinks he is “about to get wide acceptance by mainstream finance people”.

Earlier this month, Tesla said it bought $ 1.5 billion in bitcoin and would accept encryption as payment for its products. The electric vehicle maker made about $ 1 billion in profits on paper from its investment in bitcoin, according to Dan Ives of Wedbush Securities.

Tesla is “on a path to profit more from its investments in Bitcoin than profits from the sale of its EV (electric vehicle) cars throughout the year 2020,” estimated Ives in a note published on Saturday.

Bitcoin is gaining traction with conventional investors, in part due to the perception that it is a store of value similar to gold. Bitcoin optimists have attempted to argue that investors should add cryptocurrency to their portfolio to protect themselves against possible rising inflation.

But skeptics are not convinced. JPMorgan analysts said in a statement last week that bitcoin is an “economic spectacle” and that the rise of digital finance – not bitcoin – is the “true story of financial transformation in the Covid-19 era”.