By Callum Keown iStockphoto Cryptocurrency, the market that never sleeps, is once again grabbing the attention of investors. Bitcoin (BTCUSD) tumbled on Sunday, hitting a 24-hour low of $31,227 — more than 50% below its mid-April peak of $64,829. It comes amid environmental concerns over the mining of bitcoin, raised by Tesla (TSLA) Chief Executive Elon Musk, and a crackdown by China on the sector. The world’s No.1 crypto stabilized on Monday, trading above $36,000. Our call of the day comes from Michael O’Rourke, chief market strategist at JonesTrading, who says gold’s “speculativ...