A hallmark is an official seal that’s stamped onto precious metals and certifies the purity of the item. Because it’s impossible to tell how pure a precious metal item is by just looking at it, the process of hallmarking guarantees a standard of quality.
So, if a piece of gold jewellery is stamped with a ‘gold’ hallmark, the buyer knows the piece is real gold.
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Gold, silver, platinum, and most recently, palladium, must all be hallmarked by law. Gold (in its variations of white, yellow and rose gold) must also be hallmarked as 9ct, 14ct, 18ct etc standards.

This is where is gets a bit like a maths class!

Take a look at your solid gold jewellery and you’ll see it’s stamped with a number. This number tells you the purity of the gold.

24ct gold: At the top of the scale, 24ct gold is 100% pure gold.
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9ct gold: 9ct gold is 37.5% pure gold. 9ct gold jewellery is stamped with the number 375.
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14ct gold: 14ct gold is 58.5% pure gold. 14ct gold jewellery is stamped with the number 585.
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18ct gold: 18ct gold is 75% pure gold. 18ct gold jewellery is stamped with the number 750.