Yellow gold used in jewellery is not 100% pure, as it is too soft and will scratch very easily. To create a more durable alloy, manufacturers often mix yellow gold with zinc and copper known as alloys.
9ct yellow gold is hallmarked 375 which means 37.5% of the gold content in your item of jewellery is pure gold and the rest is alloys. 9ct gold is stronger than higher carat gold. The other metals in 9ct gold harden it, making it a more durable choice.
18ct yellow gold is hallmarked 750 which means 75% of the gold content in your item of jewellery is pure gold and the rest is alloys.
Due to its higher percentage of pure gold, 18K gold is quite noticeably softer and easier to scratch or dent than 9ct yellow gold but the gold content makes it a good investment