Until China came along, technological advances meant the world was using fewer and fewer commodities in manufacturing. In fact, China has been almost single-handedly responsible for the reversal in the commodities bear market that has persisted for almost 20 years, and China now accounts for 30% of world coal consumption and 40% of steel consumption.

The thing is, commodity prices have been so low for so long that even recent demand has not bolstered supply sufficiently. Adding to capacity will take time because all the easy ways of increasing supply have already been implemented, and companies are taking their time over long-term decisions, concerned that the additional capacity will come on stream just when demand drops.

So commodities and mining-company shares, which have posted sizeable gains over the last two years, may still be a solid long-term investment.

Another advantage that ‘China-play’ commodity companies have over the manufacturing or service businesses supplying China is that commodities like coal and copper are virtually impossible to counterfeit, and their producers don’t need to worry about branding or marketing.

Jim Rogers, a former partner of George Soros, is still calling a ‘buy’ on commodities and on China, and he got into the game early on both counts. And don't forget that India might take off industrially, too.

Timing is also important. The prices of commodities (and their producers) are more volatile than stock markets. Throughout the first half of this year, the commodities markets worried that China's overheating economy was in for a hard landing, which could be disastrous for commodities companies, having profited from some of the strongest global demand for resources in decades. So far, however, fears have generally not been borne out, and prices have recovered most of their losses.

In any case, a slight decline in Chinese demand may help commodities in the long run. According to Merrill Lynch, some slowing in Chinese demand could be a good thing, “because high prices always lead to too much supply being brought on the market."

Some investors are putting money directly into commodities, rather than via companies. But this is risky, even for those willing to track the prices daily. Futures and commodities are highly leveraged, very volatile and best left to professional traders, while the commodities sector is prone to geopolitical concerns and worries about disruptions to supply.