You can look up the price history for just about any diamond from these online jewelers. Simply copy the link to a specific diamond from the jeweler's website, paste it into The Diamond App, and you'll see how the price of the diamond has changed over time.
If you don't have a partiular diamond in mind, you can browse for diamonds instead. Each diamond's price history is already analyzed for you, so you can quickly see if the price has recently dropped.