Carat in gold terms refers to the content of pure gold in the item. Usually this is a 3 digit number representing the parts per thousand of pure metal. Each of these numbers translates to a carat of gold. For gold the purities are as follows:
375 = 9ct (37.5% pure gold) 585 = 14ct (58.5% pure gold) 750 = 18ct (75.0% pure gold) 916 = 22ct (91.6% pure gold)
Pure gold is 24ct but the purity mark is always stamped 999.9. This is because the slightest impurity would technically stop the gold being 1000 parts per 1000 so this mark protects the retailer/supplier.
In the UK we most commonly use 9ct & 18ct gold with some plain wedding rings being made from 22ct gold. 10ct & 14ct is usually imported jewellery that does not always come with a full hallmark. In imported, very old, or unhallmarked jewellery there is often a more literal stamp on the piece of jewellery ie 9ct or PLAT.
The image above shows a wedding ring made by RING jewellers, Brighton. The ring is a hallmarked 9ct white gold band. The ring contains the following symbols: The 375 purity mark of 9ct gold. The anchor symbol of the Birmingham assay office & lastly the Fairtrade Gold Symbol.
Although the term carat is not used in other precious metals the purities are as follows:
925 = sterling silver (92.5% pure silver) 950 = Platinum OR Palladium (95.0% pure Platinum OR Palladium)
On rare occasions Platinum comes in a 90% purity & Palladium comes in a 50% purity.