Warnings grow louder over cryptocurrency as valuations soar

With bitcoin and Ethereum gathering momentum among investors, some experts fear a bubble could soon burst

Joe Kennedy, patriarch of the Kennedy clan, said he knew it was time to exit the stock market after a shoeshine boy gave him stock tips. If everyone thinks it's time to buy, it's time to sell, reasoned Kennedy. Then came the great crash of 1929 to prove him right. Perhaps some of that thinking could be applied today to the digital currency bonanza.

In recent months, warning voices have grown louder as the digital assets known as cryptocurrencies have attained record valuations. The price of bitcoin, the most famous cryptocurrency, has soared this year, from $969 to more than $5,000 in September; rival Ethereum began the year at $8 and has traded as high as $400 - while new coins or tokens are issued weekly, often attached to tech startups as a way to raise venture capital.