Meghan O'Rourke makes the case against the diamond engagement ring, and includes this theory as to the ring's rise:
Until the 1930s, a woman jilted by her fiance could sue for financial compensation for "damage" to her reputation under what was known as the "Breach of Promise to Marry" action. As courts began to abolish such actions, diamond ring sales rose in response to a need for a symbol of financial commitment from the groom, argues the legal scholar Margaret Brinig—noting, crucially, that ring sales began to rise a few years before the De Beers campaign. To be marriageable at the time you needed to be a virgin, but, Brinig points out, a large percentage of women lost their virginity while engaged. So some structure of commitment was necessary to assure betrothed women that men weren't just trying to get them into bed. The "Breach of Promise" action had helped prevent what society feared would be rampant seduce-and-abandon scenarios; in its lieu, the pricey engagement ring would do the same.
She also mentions one of my least favorite ads, "An American couple stands in a plaza in Europe. The man shouts, "I love this woman!" The woman appears mortified. He then pulls out a diamond ring and offers it to her. She says, in heartfelt tones, "I love this man."' That always struck me as an excellent commercial for divorce, not diamonds.
