The main point of difference between the 18ct and 9ct Gold is the amount of pure Gold present in them. 18ct gold jewellery is comprised of 75% of pure gold and the remaining 25% is made up of other metals, such as copper and silver. In contrast 9ct gold jewellery contains 37.5% pure Gold and the remaining 62.5% of other metals, such as silver, palladium and zinc. Consequently 18ct gold contains three-quarter pure Gold while the 9ct one third.
Other factors that differentiate 18ct gold from 9ct is the colour. 18ct Gold jewellery, because of the significant presence of pure Gold in it, retains its shiny colour significantly longer than 9ct. In contrast, 9ct gold jewellery tends to tarnish and discolour. The reason is the low content of pure Gold and high content of other metals.
As a result, the high proportion of pure gold in 18ct gold jewellery makes it more expensive than the 9ct gold jewellery.