Diamond has the illusion of being rare, thanks to careful release and stockpiling of the material in massive underground vaults but the controlling companies. Colors diamonds and very large, well cut clear diamonds above 1 carat are apt to gain in value. The illusion that diamonds are rare keeps the price up. Diamonds over 5 carats are truly rare not just high priced and colored diamonds especially in large sizes of good clarity are truly rare.
But never fear the price of your diamonds are still steadily going up due to inventory management and careful release of the material, and marketing.
Diamond was not the most popular engagement ring gem until heavy marketing after the turn of the century popularized it. The diamond market has been flooded with Moissanite which is difficult to detect and is as beautiful, many think more beautiful than natural diamond. It is, manmade diamond virtually the same in composition as a real diamond. Like diamond moissanite can be lower and higher quality. Many people are beginning to use Moisanite and other brands of manmade diamonds in their jewelry, not wanting to pay for a stone that is rare only due to corporate finagling.