Diamond prices sometimes increase, sometimes decrease--just like everything else. Also, how much you paid for a particular diamond in the first place is important. People often overpay for diamonds. I know there is a standard chart for current diamond prices that dealers use (those prices have to be evaluated considering other factors--like how good the cut is). But I am not a jeweler and certainly not a diamond expert--lol. Minor differences in stones can greatly affect their value. From what I have seen the larger, better quality stones seem to hold their value much better than smaller lower quality stones. [Like for e.g. skyqueen says she sold a 5 yr old, 5 ct. RB for what she paid for it last year in a bad economy]. I think you need to consult an expert in diamonds regarding any specific stone you have in mind.
With that said, I would think if the stone is 18 years old, generally, it should appraise for more than it's original purchase price (assuming you didn't grossly overpay in the first place or something crazy wasn't happening in the diamond market at that time). I thought diamond prices were down a little (meaning down from a few years ago, not 18 years ago) with the economy, but I haven't heard about any major crash in their value. Your best bet is to consult a graduate gemologist who specializes in diamonds for advice on any particular stone.