This is the same slogan as chapter 90, and it appears again in this part because the copywriting is only half of the story. The other half is a supply problem.
De Beers controlled the world's diamond supply through a cartel that restricted release to maintain price. The arrangement had one structural vulnerability, and it was not competition.
It was the second-hand market.
Diamonds are not rare. They are common, and their price is a function of controlled supply and manufactured demand. If a substantial resale market had developed, buyers would have discovered two things simultaneously: that a diamond bought for a thousand dollars would fetch a few hundred, and that there were a great many of them.
A Diamond Is Forever solves this. If the stone is an inalienable symbol of a marriage, it is not an asset and selling it is not a transaction but a statement about the marriage. The resale market never formed, and the retail price was never tested against what anyone would actually pay for a used stone.
Two psychological mechanisms support the frame.
The endowment effect of chapter 17: once the object is coded as mine, and in this case as constitutive of a relationship, parting with it is processed as a loss rather than as a foregone gain, and the asymmetry is roughly a factor of two even for a coffee mug.
And commitment signalling, which inverts ordinary price sensitivity. In most purchases a higher price is a cost to be minimized. Here the expenditure is the message — the whole point of a costly signal is that it is costly — so De Beers could publish guidance on how much a man should spend, first one month's salary, then two, then in Japan three, and the guidance functioned as a floor rather than as an imposition. Spending less is not thrift; it is a statement about how much she is worth to you.
Social enforcement completed it. Each couple that complied raised the descriptive norm for the next, and the ring is worn visibly on a specified finger where compliance can be observed.
Edward Jay Epstein's 1982 Atlantic article, Have You Ever Tried to Sell a Diamond, laid out the mechanism in full, including the resale problem and the deliberate campaign against secondary sales. It has been in print for forty years, and the norm has not moved.
Cross-ref: Attack — Manufacture Scarcity