Go after the cultural norms with celebrities. Get Oprah and Ellen and Kate Middleton and Cosmo and Glamour on board, and arrange for high-profile weddings to have manufactured diamonds and large charitable donations. Things like this can change pretty fast when it's made obvious that there really is a right side and a wrong side. Gay marriage is a recent example, and that has the extra hurdle of having to overcome strong religious beliefs.
But you'll never replace diamonds with manufactured diamonds. The whole point of them is they are expensive.
Expensive, scalable (huge diamonds for celebrities, small ones for poor people, and everything in between), durable, small enough to slip in your pocket (to surprise them), and backed by a monopoly supplier.
You could try it with another rare gem (rubies, emeralds, opals) but you won't have a monopoly supply, so you can't compete with the advertising dollars of the diamond miners.
You could try to switch the marriage tradition to something completely different (look at China, where are house and car are generally a pre-requisite for marriage), and hope that the practicality of the tradition wins over big advertising dollars. The problem is, celebs don't care about a new car the same way a newly married couple do.
Or you could try to encourage an "experience" gift. Paris could set itself up as the city to propose in, and smaller cities could also compete.
It's just hard to think of everything with all the advantages of diamonds. The disadvantage is their lack of practicality, and their slightly unethical origins, but nothing seems to have displaced them so far.
