Diamonds have been at the heart of myth and legend since their discovery. Medieval knights wore them uncut on their armor in the belief that they could make a person invincible, a myth no doubt related to the stone's hardness. There are legends of a diamond that could reveal the guilt or innocence of a person. Diamonds could also drive away the devil, and in the days when gems were believed to cure disease and ill fortune, diamonds were thought to amplify the magical powers of other stones a person wore.
Almost universally, diamonds have been associated with virtue, purity, strength, wealth, power, and love - and, not surprisingly, diamonds have been associated with sex, too. So it was a small jump for diamonds to become the modern symbol of love - diamond wedding rings have been popular for hundreds of years. They were believed to ensure fidelity and strengthen emotional bonds. Today, they are the preferred gift for all manner of romantic occasions.
There is one important footnote to the magic of diamonds: the magic was lost if the stone was acquired by purchase. When found or given as a gift, however, it would convey its power to the recipient, which no doubt accounts for how diamonds became such a popular gift between lovers.