Cryptocurrencies are on the rise again, helped by optimism surrounding the possible introduction of a spot bitcoin ETF. But no matter how strong this rally gets, it won't convince one of the industry's biggest critics — Warren Buffett. Bitcoin surged 10% over the past week, at one point briefly topping $30,000, as hopes grew that a spot bitcoin ETF will soon be approved. But whether a bitcoin ETF soon becomes reality or not, it won't alter the Oracle of Omaha's thesis on cryptocurrencies, which is that they are highly speculative, non-productive assets. Buffett, who once famously called bitcoin "probably rat poison squared," has compared the bitcoin phenomenon to the tulip bulb mania in the Netherlands in the early 1600s .

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Persons: — Warren Buffett, Bitcoin, Buffett, It's, Buck Rogers, Charlie Munger, Munger, bitcoin Organizations: CNBC, Berkshire Locations: Netherlands, Berkshire, U.S, United States, bitcoins