White gold is a mixture of pure yellow gold and other metals that make the resulting alloy harder and give it a white colour. Usually, the additional metals that can be used in white gold are palladium, silver, nickel, copper, and zinc. White gold has a rhodium plating which gives it the white colour and shine. This plating does wear but can be replated by a working jeweller for a small charge. Rhodium plating is also hypoallergenic.
9ct white 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. It will have a nice white finish due to the rhodium plating
18ct White 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, 18ct gold is quite noticeably softer and easier to scratch or dent than 9ct yellow gold but the gold content makes it a good investment. It will have a nice white finish due to the rhodium plating