The practice of giving a diamond ring to mark an engagement started in the 1950's (prior to that only one ring was given, the wedding ring), and giving an engagement ring to the woman you love has rapidly become one of the rituals of getting married. Famous diamond retailers such as Tiffany's promoted diamonds as a symbol of love, and of course the Marilyn Monroe movie, "Diamonds are a Girls Best Friend" cemented the association of diamonds with love.
Before the 1950's, diamond engagement rings weren't a thing thugh exchanging rings made of iron or gold date back to medieval times..
The whole concept of a diamond engagement ring was popularised by the De Beers diamond company in an advertising campaign in 1947. The De Beers slogan was "Diamonds are forever", an emphasis on the enduring quality of the stone. This was them amplified by the Shirley Bassey song from the 1970's for a James Bond movie, Diamonds are Forever.
Diamond engagement rings are a testament to the power of advertising.