Jean Louis, Since 2009, trader, buyer, seller of scrap and recycled gold
When gold is more pure, it is closer to its original hardness of 2.5. If it is mixed with copper, that has 3.0 hardness, it is harder.
Gold of 21 karat is 87% of gold. The calculation is simple 21 divided by 24 = 0.87. Gold of 22 karat it 91% of gold, as 22/24 = 0.91. Because there is less gold in 21 karat gold, such alloy would be assumed to be harder. But it is not necessarily so. We cannot at all conclude which one is harder, as we do not know what is the other metal in that alloy with gold.
If the other metal is silver, the hardness may remain, I suppose so, at 2.5 regardless of the ratio of silver and gold, as both silver and gold have the hardness of 2.5.
If there is some harder metal inside, even 21 karat can be harder than 22 karat, as that all depends of that other unknown alloys.
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