The diamond engagement ring is routinely described as an ancient custom. Its modern form is a documented advertising campaign.
Diamond rings as betrothal tokens do have earlier precedent among European nobility, conventionally traced to the ring Maximilian of Austria gave Mary of Burgundy in 1477, and diamonds appear in aristocratic marriage gifts thereafter. But diamond engagement rings were not a general practice, and in the 1930s the diamond market was weak and the stone was not the default.
De Beers, which controlled the great majority of world rough diamond supply, engaged the advertising agency N. W. Ayer in 1938. The campaign that followed created the slogan A Diamond Is Forever in 1947, promoted the idea that a man should spend a defined multiple of his salary on a ring, a figure that was itself raised over the decades, and placed diamonds systematically in film and on public figures. The proportion of American brides receiving diamond rings rose from a minority to a large majority over the following decades. The campaign is one of the most studied in the history of advertising and De Beers has never particularly disputed the account.
The permanence slogan also solved a specific commercial problem: a stone that is never resold has no secondary market to depress prices. The scarcity, likewise, was substantially managed rather than natural, through supply control that persisted until the cartel's structure weakened in the 2000s.
Vedic tradition assigns diamond, heera, to Venus, Shukra, and it is one of the navaratna. That attribution is genuinely older than the advertising, comes from a different civilisation, and has nothing to do with marriage in the Western sense.