Good diamonds depreciate less than say cars or stereo equipment or computers. A Tiffany's ring with GIA certs can resell for half its original purchase price. There is an active secondary market in diamonds, so if you buy a high-quality GIA certified stone used, you should be able to sell it for a similar price to what you bought it for. Most of the depreciation on a diamond happens the moment you "drive it off the lot" so a used diamond is going to retain its value much better than a used car.

The key though is having a good quality stone from a reputable cutter with full certification. Diamonds are not fungible like say gold. A low-quality diamond purchased in an expensive mass-market ring isn't going to be worth anything on resale.