Diamond generally keeps its value stable over the time, at least much more than precious metals (whose prices have high volatility even in the short term). On the other hand, the lack both of an official pricing and a reference market for resale make the diamond much less liquid than precious metals, and often its liquidity is possible only with a high discount on the price.
Furthermore, for the evaluation of the diamond it is not enough just considering the fineness (as for precious metals) but it must be also considered the weight, the color and the cut.
Diamonds are traded in the main diamond exchanges all around the world, the most important of which is in Antwerp, Belgium. However, unlike the main precious metals (and in particular gold), there is no official pricing for diamond, as every trade in the diamond exchange takes place in the private negotiation formula.