We are often asked the question about which is the harder metal, 9ct or 18ct gold, the answer (as always) is not particularly straight forward.
Gold as a pure element is a very soft metal, and so theoretically, when it is alloyed with other metals to make 18ct or 9ct it becomes harder and therefore more durable. Some argue that as 9ct is only 375 parts of gold per thousand, and 625 parts alloy it is the hardest. However, it may not always be this simple as the metal the gold is alloyed with affects its hardness too. Gold is often alloyed with a combination of silver, palladium, copper, nickel, zinc and platinum.
Silver is also very soft, whereas platinum and palladium are much harder metals, for example.
Different companies use different percentages of the alloys, the two consistent elements are the gold content and nickel content (the guidelines on nickel content are very strict). Therefore, the hardness of 9ct and 18ct tends to vary from item to item.
Our advice is just to always match the gold fineness of pieces of jewellery that are to be worn together. For example, an engagement ring and wedding ring would both be 9ct of 18ct or even both platinum. This helps to ensure they wear against each other more evenly.