If Gold isn't money then why are all the Central Banks buying it?

Here is an interesting exchange between Ron Paul and the Bernanke during a July 13, 2011 appearance before Congress:

Paul: Do you think gold is money?

Bernanke: (pause) … No.

Paul: Even if it’s been used as money for 6,000 years … somebody reversed that? Eliminated that economic law?

Bernanke: Well, it’s an asset. Would you say treasury bills are money? I don’t think they’re money either.

Paul: Why do central banks hold it? Why don’t they hold diamonds?

Bernanke: Well it’s tradition. Long term tradition.