It is believed that diamonds are created under extreme conditions of heat and pressure.
The carbon atoms bond together to form crystals under this high pressure and therefore just like these conditions, they are rare to find.
Now, recently scientists did invent a way to make diamonds under artificial conditions, though these synthetic ones have a much lower value, often being 15% to 30% lower in price.
Still, diamonds remain valuable as their perfect shape and form makes them very desirable. As long as people desire jewelry that is adorned with diamonds, so will such items retain high price.
This is the law of demand.
There is also another, less expensive way of having diamonds for less money. They are made in laboratories all around the world that make diamonds by applying hudge pressure to carbon and they are as beautiful as the real ones are without the doubt that some kid worked in the deep hole to get it. They cost much less, and they are original.
They are rare and many people work hard to get them out of the ground. The transport of legal diamonds is long and after that diamonds have to be cut to have a perfect shape. It is the hardest material on the world.