Believe it or not, the one and the only reason why diamonds are considered special is that they were well-marketed by the De Beers Group. In fact, until the De Beers Group came along, diamond rings were not considered as romantic as they are today and diamonds were not even considered special.
In 1888, after De Beers became a merged institution (cartel), De Beers gained and controlled most of the production and distribution of diamonds, especially in Africa. Thus, De Beers needed a good marketing strategy to sell all these diamonds. One of its marketing strategies was to perpetuate the illusion of scarcity of diamonds, which helped them in stabilizing the prices of the gem.
In order to sell diamonds, De Beers needed a marketing strategy to fulfill their vision:
To sell more and bigger diamonds (people in the USA were spending more on low-quality diamonds).
To prevent people from reselling diamonds (Diamonds were already plentiful and hence had less resale value).
Give it a purpose, to convince people to buy diamonds (which should last long).
Therefore, the De Beers Group decided to connect diamonds with love by advertising that the bigger the diamond, the more you love someone and that you should propose to your partner with a diamond ring.
The poster above is just one of the marketing strategies De Beers used. After countless advertisements, the demand for diamonds quickly grew, and the De Beers group heavily benefited from the production of diamonds, since they own most of the diamonds in existence.
At first, diamonds were just for the rich, but after the great depression which resulted in a great fallback, it forced them to look for ways to maintain and create a demand.
They, once again, needed a new marketing strategy to put diamonds in use forever, and now, diamonds are considered special, rare, romantic, and expensive.