As Kamhaji points out, the nice thing about diamonds is that once you buy them, you don't have to do anything to maintain them; you just need to keep your investment safe. A diamond isn't going to grow, change, etc. So you can see why it's really important to do research before buying so that you know you've purchased something that will give you a good return on your money.
Kamhaji is one of many diamond experts and traders out there sharing information with investors. With just a few clicks online, you can find out a lot of information about diamonds and what types have traditionally been strong sellers. Personally I've been glad to see that more and more consumers emphasize the importance of buying conflict-free diamonds, and Kamhaji also advises that you be sure to get the complete paperwork on any diamond you buy.