Fans of bitcoin, a crypto-currency, have long called it digital gold. Now this sounds like an insult: continuing its stellar rise, and adding more than 30% to its value in just a week, one bitcoin is worth more than $2,600, over twice as much as an ounce of gold. As The Economist went to press all bitcoins in circulation were worth over $43bn. A sum of $1,000 invested in bitcoins in 2010 would now be worth nearly $36m. Other crypto-currencies are also marching upward: together this week they were worth $87bn. But if the history of gold is any guide, what goes up will come down—and then go up again.