The golden rule
Most people believe, or have been told, that 9-carat gold is harder, or harder wearing than 18-carat gold.
I sometimes have trouble convincing them, that in fact 18ct gold is the winner. There are many different ‘recipes’ for 9ct and 18ct gold alloys. You would be correct to guess that each different recipe has its own different characteristics, including hardness.
It is quite possible to make hard or soft recipes for 9ct and also for 18ct and other gold alloys.
In general most common 18ct gold alloys are both harder and harder wearing than their 9ct equivalent.
This brings me to the point that hardness and durability are not one and the same thing. To give an example, a glass ball is harder than a rubber ball. Try throwing each onto a hard surface. The glass ball will break, but the rubber ball will bounce and remain intact, because the rubber ball is more durable than the glass one. The glass ball breaks because it is brittle. In the same way, metal alloy can also be brittle, and hard nine carat gold alloys tend to be slightly brittle, whereas 18ct gold alloys tend to be more resilient.
18ct alloys are almost completely resistant to chemical attack in normal use, whereas 9ct alloys are much less resistant.
For example, 9ct alloys will go dull or even black merely from exposure to chemicals in the atmosphere.
They will also discolour in contact with perspiration, some fabrics, bleach and other household chemicals.