Diamonds are traditional for engagement rings and so is spending three months' salary on them.
The idea that you should spend an enormous amount of money on a rock for an engagement ring can be traced back to a 1938 advertising campaign by the diamond sellers DeBeers.
It was this campaign, primarily targeted at men, which helped create a sense of luxury and tradition around diamonds. They're actually not as rare or as valuable as you'd think
Men buy over 90 per cent of all engagement rings, so they needed see that diamonds were a gift of love and that the size and quality of the diamond was an expression of that. Young Women needed to associate this as integral to a proposal.
The problem was, demand wasn't there for a stone that young men couldn't afford and was not seen as integral to a proposal.
So, DeBeers began to market them as a status symbol, on the advice of Gerold Lauck of the N.W.Ayer advertising agency:
The substantial diamond gift can be made a more widely sought symbol of personal and family success — an expression of socio-economic achievement.
The convention was thus fabricated that a man of means and aspiration would buy his wife a diamond ring - anything less would be a disappointment - and it weirdly holds to this day.
These days over 80 per cent of women receive a diamond ring when they get engaged. The market remains convinced.
It remains, however, a poor investment. Diamonds have far less inherent value than they retail for.
Prices have been driven up by the fact that a few companies held dominance over the diamond market for decades.
If you try to sell a diamond you own to a jeweller they will likely offer you a third of the price you bought it for - the ones in store are still marked up 100 or 200 per cent.
It's a poor investment and even poorer expression of love - your gesture of commitment is simply to adhere to a tradition unthinkingly; because some ad men told your grandparents it's what they wanted.