JPMorgan's top quant strategist backed his boss this week in bashing bitcoin, warning that the cryptocurrency is likely a "pyramid scheme."

Marko Kolanovic, the bank's global head of quantitative and derivatives strategy, said in a note on Wednesday that in addition to being volatile and difficult to value, "another worrying aspect of cryptocurrencies are some parallels to fraudulent pyramid schemes."

Bitcoin needs to be mined, or discovered, by people using computers to solve problems. Its murky origin raises questions, he said. "It is believed that an unknown person (or persons) known as 'Satoshi Nakamoto', before disappearing, mined the first 1-2 million coins or about 10 percent of the coins that will ever exist."

He added, "Mining becomes progressively more difficult, and eventually unprofitable."

Bitcoin surged more than 10 percent on Friday, but was still on track for a big weekly loss during a tumultuous period of trading.

Earlier this week, JPMorgan CEO Jamie Dimon called bitcoin a "fraud," saying that the cryptocurrency "won't end well." Dimon was speaking at the Delivering Alpha conference presented by CNBC and Institutional Investor.

Bitcoin plunged about 13 percent Thursday after one of the biggest exchanges in China said it will shut down its operation. BTC China said in a tweet Thursday that it will close down its operations by Sept. 30 as Chinese authorities crack down on cryptocurrencies.

But to be sure, many see bitcoin as a huge opportunity.

Former JPMorgan strategist Tom Lee said the cryptocurrency could surge another 600 percent in five years. "I unequivocally believe bitcoin is your best investment to the end of the year," the Fundstrat co-founder told CNBC, standing by similar remarks he made Aug. 9 on "Fast Money."

"It's not worth it to look at bitcoin two months, two weeks ahead," Lee argued, saying he still believes each bitcoin will be worth $25,000 in five years.

An unconvinced Kolanovic rattled off a number of risks. "There is no organized power behind this currency to e.g. ensure its long term viability, secure trade, enforce its acceptability for goods and services, or provide investor fraud protection," he said.

"If the use of cryptocurrencies were to increase to an extent that they start competing with traditional 'country' currencies they would be quickly regulated or outlawed."