9ct Gold is Harder? Unfortunately you would be misinformed!
Most people are surprised to hear this, because for some reason, they have always believed, or been told, that 9 carat gold is harder, or harder wearing than 18 carat gold. We sometimes have trouble convincing them, that in fact 18ct carat gold is the winner.
As usual, the answer to a simple question “which is harder, 9ct or 18ct gold?” is not always simple.
There are many different combinations of metals which can be used to make gold alloys. There are many different "recipes" for both 9 carat and 18 carat gold alloys. You would be quite 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 9 carat and also for 18 carat and other gold alloys. In general, most common 18 carat gold alloys are both harder, and harder wearing than their 9 carat equivalent.
This brings is to the point that hardness and durability are not one and the same thing. To give a simple 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 at 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 9 hard carat gold alloys tend to be slightly brittle, whereas 18 carat gold alloys tend to be more resilient.
18 carat alloys are almost completely resistant to chemical attack in normal use, whereas 9 carat alloys are much less resistant. 9 carat alloys for example 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.
To give you an indication of the hardness figures for 9 and 18 carat golds, the following table gives a range of typical "Vickers" hardness values for fairly common "recipes":-
Alloy Maximum Annealed Hardness
9 160 to 170
14 150 to 180
18 170 to 230
22 60 to 90
To finish with a very simple personal opinion. If there had to be only one single gold alloy purity standard, then I believe that 18 has such excellent all round properties that it would deserve to be the winner.