Fine gold or 24ct is too soft for most jewellery pieces so we add alloys to make the metal harder.
18ct is 75% fine gold and 25% alloys (e.g. silver, copper, and palladium) whereas 9ct is 37,5% fine
gold and 62.5% alloys. As such 18ct is the purist alloyed gold that is perfect for making jewellery
with. 9ct is a cheaper metal but with a lot less fine gold and a lot more alloys. You would assume that
as there’s less fine gold in 9ct that it would be a lot harder than 18ct, but the difference is very small.
It has more of an impact in manufacturing as 9ct is more brittle and more difficult to shape and solder.