What is more valuable gold or diamonds?

Gold.

Gold is the most liquid of all, even cash. cash will loose value due to inflation. Diamonds do not have an absolute value - different valuers will value a particular piece differently. Gold on the other hand has an absolute value - which is generally not influenced by the exchange rates.

If in a fit of need you will want to sell a diamond - you will not get its full value - but only the salvage value. if you need to sell gold you will get the full value.

Another Answer

Given exact weights of each element, diamonds will always be more valuable, in that its market price will always be higher.