Credit default swaps: were initially created as a way for banks to manage their exposure to credit risk. They were essentially insurance contracts against the default of a borrower defaulting so they played and to this day play a huge role in mitigating risk -- their use in speculation etc.. only became prominent after 2008.
Collateralized debt obligations: created as a way to pool and securitize various types of debt, providing a means for financial institutions to manage risk and enhance liquidity. Just because a few banks and institutions used them to create complex tranches of debt which 'attempted' to reduce risk while massively misusing statistics and increasing it which resulted in 2008 doesn't mean that CDOs aren't useful.
Gold: I suppose you haven't heard about its applications in electronics and connectors, bonding wires, connectors, switches, thin film applications and coatings and hugely used in the production of medical devices and in the aerospace industry?
Diamonds: this one I agree semi-agree with. The only reason they have value is because the De Beers limited supply and there was demand -- but the demand is there because people use them as a status symbol and men use them to show their 'love' or commitment to their partners so this one is questionable.