Diamonds may be not so tough after all. While investors have been piling into commodities of all sizes, the market for diamonds is showing signs of cracking. That might seem odd considering, like other resources, supply trails growth in demand. But diamonds remain tricky to find and expensive to extract – medium-term supply growth is just over 1 per cent per year, with mines in Canada and Africa just about offsetting declines in Australia. Demand, on the other hand, has been growing five times that, lifting average prices by about 30 per cent since 2003.

But two negatives loom large. The first is that, unlike the copper being gobbled up by China, 50 per cent of all gem diamonds still end up in the US. Girls across the developing world, and even Europe for that matter, are not ditching their existing best friends as brutally as hoped. And although love may endure, diamonds are discretionary – falling confidence in the US has already affected the market for smaller diamonds. Traders are worried demand is softening for larger stones too.