(I was actually surprised the engagement ring thing started with following royalty, not marketing.)
In Western culture, diamonds are the traditional emblem of fearlessness and virtue, but have also often associated with power, wealth, crime and misfortune. Today, diamonds are used to symbolize eternity and love, being often seen adorning engagement rings and sometimes wedding rings as well. The popularity of this modern tradition can be traced directly to the marketing campaigns of De Beers, starting in 1938. The diamond engagement ring is, however, not an original invention of De Beers. It can be traced to the marriage of Maximilian I (then Archduke of Austria) to Mary of Burgundy in 1477. Other early examples of betrothal jewels incorporating diamonds include the Bridal Crown of Blanche (ca. 1370-80) and the Heftlein brooch of Vienna (ca. 1430-40), a pictorial piece depicting a wedding couple. Inaccessibility of diamonds to the vast majority of the population limited the popularity of diamonds as betrothal jewels during this period.
Diamonds were also a symbol of gay community in the 1950s. The Mattachine Society, one of the first and the foremost gay rights groups in the United States, used so-called harlequin diamonds (four smaller diamonds arranged in a pattern to form one larger diamond) as their emblem.
The LifeGem company further taps modern symbolism by offering to synthetically convert the carbonized remains of people or pets into "memorial diamonds". However, many people feel very uncomfortable at the thought of wearing the carbonized remains of people as jewelry.