Bitcoin retreated Tuesday after a bout of volatility highlighted lingering doubts about the durability the token’s mesmerizing rally. The cryptocurrency fell about 6.8% to $51,210 as of 11:52 a.m. in Tokyo on Tuesday, following a drop of as much as 17% a day earlier to below $50,000. The token is still up more than 400% in the past year, a rise that dwarfs other assets. Treasury Secretary Janet Yellen and Microsoft Corp. co-founder Bill Gates were the latest to weigh into a debate over the digital coin. Gates cautioned about how investors can be swept up in manias, while Yellen said Bitcoin is an “extremely ... (full story)