Diamonds are a commodity item, created by years of advertising by deBeers and their crime partners. They are not nearly as rare as you have been led to believe. deBeers controls the market, and holds back on inventory to create artificial scarcity. Also, there is now technology to created diamonds from carbon. They are technically indistinguishable from so-called “natural” diamonds, but are much cheaper. To buy a diamond is to buy into a structure of trickery and myth.
My suggestion is to look for a lovely piece of well-made Estate jewelry that contains pleasant gems. Old-fashioned rosecut diamonds in a beautiful Victorian or Georgian setting purchased at auction at a major house such as Sothebys, Christies or Phillips will never lose value, and may in fact appreciate.
While part of the value will be in the stones, most of the value will be in the skill of the jeweler who made the setting. No decent jeweler used crap stones, so you can be assured that the diamonds are reasonably good, they will be old, they will be beautifully set, and they will not be a part of the hustle.
You need not attend a “Fine Jewelry” sale and spend hundreds of thousands of dollars. Go to a pleasant Sothebys Arcade jewelry auction where the lots are priced at $10,000 and below, and ask the curating expert to help you choose. If you are polite and pleasant and explain that you are no an expert, but are seeking a beautiful gift at a certain price, they will be more than happy to help you select a few items to bid on. If you don’t get what you wanted the first time, wait for the next sale. This time the attending expert will remember you and will be even more pleased to help, and you will end up owning an heirloom that you can pass down in your family.