Bitcoin has climbed more than 77 percent so far in December alone. : Bitcoin blasted to another all-time high of almost $18,000 on the Bitstamp exchange on Friday, up 9 percent on the day, as warnings grew over the risks of investing in the highly volatile and speculative instrument. The cryptocurrency's staggering recent price rises -- more than 1,700 percent since the start of the year -- have driven worries that the market is a bubble that could burst in spectacular fashion.