Investopedia is a great reference tool, particularly for those new to finance. But once in a while it provides advice that may not be entirely accurate. In an article entitled "Commodities: The Portfolio Hedge" Investopedia tells you why commodities help "diversify" your portfolio.
Investopedia: Commodities tend to bear a low to negative correlation to traditional asset classes like stocks and bonds....
Yes, it is true that the average correlation between equities and commodities from 1970 to 2003 has been negative. But if we've learned anything from the financial crisis, it is that correlations are not static and tend to spike in a deleveraging environment (whether you are dealing with mortgages, equities, corporate bonds, etc.). The chart below shows the correlation between the CRB Commodity Index (or equivalent basket of commodities) and US equities going back to 1915 - almost a century of historical data.
In a crisis, a commodity basket may not be very effective in providing the diversification one would expect from historical correlations (reaching 80% in the recent crisis as commodities sold off with equities). The same thing happened in 1929. There is nothing wrong with having commodities in a portfolio of assets. One simply needs to be aware of the reasons the asset class is part of it. And the key motive to be long on a broad basket of commodities is to protect against inflation, not to try "diversifying" the portfolio.