Wiktionary
diamonds are a girl's best friend
prov. A statement that suggests, while love is a luxury, material wealth (particularly jewellery) is more valuable to a woman.
Wikipedia
Diamonds Are a Girl's Best Friend
"Diamonds Are a Girl's Best Friend" is a jazz song introduced by Carol Channing in the original Broadway production of Gentlemen Prefer Blondes (1949), which was written by Jule Styne and Leo Robin. It was based on a novel by Anita Loos.
Usage examples of "diamonds are a girl's best friend".
I don't know if you've heard this or not, but there's a saying that man's best friend is a dog and that diamonds are a girl's best friend.
A rock on the head may be quite sentimental,' she went on, the certainty draining out of her voice as she surveyed the sentence ahead of her, 'but diamonds are a girl's best friend.