Cartier describes its precious metals in terms of millesimal fineness, rather than the karat system.
Millesimal fineness can be used to describe all metals and it measures the parts per thousand of pure metal in each alloy.
For the Maison, they are as follows:
Gold (white, rose, yellow): 750/1000
Platinum: 950/1000
Palladium: 950/1000
Silver: 925/1000
The karat system, which can be used only for gold, measures the parts of gold per 24, i.e. 18 karat gold is 18 parts gold per 24, therefore it is 75% gold. 24 karat gold is 100% gold.