Well if cash can be a good diversifier, so can gold, even more so, if you don't mind the added volatility of the yellow stuff.
Gold has been the key association for anything related to riches and wealth for a very long time. And even though it has lost some of its allure it is still an very actively traded good.
By definition you will not make any returns on holding gold other by selling at a higher price then you have brought it. There is no appreciation of the metal itself, and no interest payment either.
If you are actually buying bullion as an investment you either have to store it with your local bank, which will cost you, or you take it home and hide it under your mattress.
As an alternative many people buy funds showing the expression somewhere in their description. These funds usually invest their money in gold and other precious metal mines, i.e. in the production of the metal rather than the metal itself.
On both the Australian Exchange ASX and on the London Stock Exchange LSE are ETF's traded which are based on the real metal, and iShares have been planning to introduce a gold-based ETF for some time now as well.
Whether you are going to introduce gold as an diversifier to your personal portfolio is really up to your personal capacity to carry risk, and probably even more to the extend that owning some bullion soothes your nerves.