^^ I am a bit hesitant to respond b/c i dont want to come off like i am in any way an expert..bc i am not at all! i do not have any detailed research to back up what i say, and all i offer is personal experience.
with that said, i know for a fact that if you want to RE SELL a 2.5 carat (G colored, VVS, Ideal cut ROUND) solitaire, you can make about 30% MORE than what you paid for about 5 years ago. I know this for a fact b/c one of my closest girlfriend did this @ the diamond district in NYC (by Rockefeller Center). If anyone wants the actual store name, I can provide that too..just PM me..lol..I actually accompanied her during my lunch break b/c she didnt want to be alone.
Therefore, I do not think diamond appreciating in price is a myth at all, because I witnessed it. Granted, her diamond was EXCELLENT (color, cut, clarity, everything).