Market value varies on everything--that's why it's called market value. Paying retail and expecting to sell at that is rarely a good idea. That said, diamonds are a poor candidate since their value is completely and artificially set by the DeBeers cartel.
Anything you hold for contingency commerce is not an investment, but a store of value. That is, something to liquidate to good purpose--it's value dependent on time, opportunity, and need.
You may not want to sell grampa's gold watch today for a measly $500, but you would think nothing of trading it for a couple of bottles of antibiotics when your child really needed them.