What exactly is a diamond? It's just a crystalline form of pure carbon, usually colorless, mostly used for cutting in heavy industry. and not worth more then a few pennies in "real" value.
The problem is that only a certain amount of commercially cut diamonds are allocated to the public each year, which creates a "hyper value" and strong market/demand for them. It's one of the truly last monopolized markets still around today. If DeBeers and other companies decided to "flood" the market with even 50% of the diamonds they're sitting on, your diamond would be worth pennies. Emeralds are more rare and there is no monopoly on them anywhere near like the diamond industry. In fact, other gemstones are rarer then diamonds.
Diamonds were so plentiful in South Africa & Australia, that hundreds of people were hired to walk in a straight line and pick them up from the surface of the ground. There are countless pictures of this. DeBeers was smart in hording diamonds and then sitting on them until the market value increased to the point where they skyrocketed in value.
DeBeers genius was that they only "trickled" out a few diamonds onto the market place, and hence, we have what we have today--which is essentially a hyper market for a piece of fossilized carbon which is essentially just a rock, nothing more. The diamond market has always been a war of supply vs demand.
There must be infinite amounts of better ways of showing one's love for someone then throwing a lot of money at a piece of carbon.
Nevertheless, good luck.
Frank