“Diamonds are a girl’s best friend” is a phrase often thrown around to prove that women are materialistic, but if you listen to the song you realize that it’s really about how men think a woman’s looks and youth are the only valuable assets she has. They’ll trade her in when she “loses her charms”, so she needs to collect diamonds and valuables to sustain herself when she’s old and alone.
Consider that the song was written and performed during a time when women couldn’t find a job for decent pay, that the only way they could survive financially was to tie themselves to a man and hope he’d stick around and provide support.
The real message is that men are shallow and flighty, so take their sweet words with a grain of salt and make sure you get something real out of the relationship.
But actually not diamonds, because they’re actually only semi-precious and DeBeers has been artificially inflating the price for ages, other precious stones will probably hold value better in the long run