"Bad news is good news" is a meme of this quantitatively eased era. But it was ever thus in commodities markets. Prices for things like fuel and gold have often jumped at the prospect of mayhem descending.

So, this week, as it looks like US military strikes in Syria might not happen, commodities have sold off sharply. For investors, this serves as a timely reminder of what has been supporting prices this summer-and what hasn't.