Why do I need to know croqueuse de diamants?

Because it's a term that you can't easily translate.

What does it mean?

Croqueuse de diamants literally translates as 'diamond biter' and is the French equivalent to gold digger.

French online dictionary l'Internaute defines it as “femme fatale avide d'argent” – a femme fatale greedy for money.

Croquer is the French verb for ‘to bite’ into something and une croqueuse is a (female) ‘biter’. So une croqueuse de diamants is someone so fond of money that she figuratively eats diamonds.

The male version is croqueur des diamants although you'll hear that a lot less because, well, the patriarchy.

Origins

Une croqueuse is an old slang term for a prostitute that “enchaîne les passes“, according to l'Internaute, meaning she takes on one customer after the other.

Together with croquer de dot, a French expression that refers to spending a dowry or heritage very quickly (burning through it), croqueuse was the base for what in the 20th Century became croqueuse de diamants.

In other words, there's no reading between the lines here: une croqueuse de diamants is a woman using her body to spend a lot of money very quickly.

(So even if the expression itself is funny, it's pretty sexist and not a very charming thing to say to someone.)

Use it like this

Franchement il n'y a rien qu'elle aime autant que d'argent. C'est une vraie croqueuse de diamants. – Frankly there's nothing she loves as much as money. She's a real gold digger.

Tu as vu comment elle se comporte avec les vieux riches ? On dirait une croqueuse de diamants. – Have you seen how she acts with old rich men? Looks like a real gold digger.