Saudi Arabia's oil giant Aramco started trading on the Saudi stock exchange for the first time on Wednesday, after a mammoth $25.6 billion IPO, that set a record as the biggest ever in history. The state-owned company had announced a sale of the firm's 1.5% stake, at $8.53 per share. At this price, Aramco is the world's most valuable company at 1.7 trillion US dollars. That's more than the world's top five oil companies - Exxon Mobil, Total, Royal Dutch Shell, Chevron and BP - combined.