I've taken a selection of single-commodity ETCs, plus the broad DJ-AIGCI tracker from the ETF Securities stable and compared yesterday's closing prices with the year's highs, showing the dates of the highs and the respective declines in percentage terms.

The declines have been spectacular, with the broad DJ-AIGCI index down 44% in little over three months, and silver, crude oil, and natural gas all down over 50% from their highs. Gold is a relative standout, falling by only a quarter since its March peak, as the financial turmoil has driven investors into the metal.

There's a visible double top in prices - most of the individual commodities peaked in March/April, with crude oil, natural gas, aluminium, soybeans and the broad index hitting highs in July.

ETF Securities' assets have fallen too - from a peak of around $10 billion to just over $7 billion last week - though by less than the broad index, reflecting continuing client inflows.

Is this a buying opportunity? If you're in the Jim Rogers camp, we're still in a bull market for commodities, so this is indeed. The bulls' argument must be that the steepness of the recent decline reflects deleveraging and the raising of cash by investors.

One relative value trade catches the eye. The gold/silver ratio - which in many countries was fixed at 16 for centuries - is at historic highs of over 80 (as was recently pointed out by Brad Zigler at Seeking Alpha) making silver look extremely cheap on a relative basis.