Bitcoin burst out of the shadows in 2017, seducing Wall Street and individual investors alike even though many still struggle to understand precisely what it is.

The cryptocurrency's rise is also pushing regulators to consider taking action after years of simply urging caution.

Bitcoin started the year at $1,000 per unit in January but by mid-December had shot to within striking distance of $20,000, a dizzying climb that stoked fears of a bubble even in financial circles used to speculation and volatility.

"Bitcoin remains a major gamble as it is very much an uncharted-waters asset," said Nigel Green of investment firm deVere Group. "An asset that goes almost vertically up should typically raise alarm bells for investors."