A fascinating twist to the story of the diamond engagement ring:
A now-obsolete law called the "Breach of Promise to Marry" once allowed women to sue men for breaking off an engagement. Back then, there was a high premium on women being virgins when they married — or at least when they got engaged. Surveys from the 1940s show that roughly half of engaged couples reported being intimate before the big day. If the groom-to-be walked out after he and the bride-to-be had sex, that left her in a precarious position. From a social angle, she had been permanently "damaged." From an economic angle, she had lost her market value. So Breach of Promise to Marry was born.
But in the 1930s, states began striking down the "Breach of Promise to Marry" law. … Regressing the percent of people living in states without Breach of Promise against a handful of other variables — including advertising, per capita income and the price of diamonds — [legal scholar Margaret Brinig] found that this legal change was actually the most significant factor in the rise of the diamond engagement ring.