Gold is gold and it is as hard as it is. Studies have shown that with all things being equal, the gold content of an alloy does not seem to affect its durability. What you are talking about are the alloys used in karated gold.

The three most common and abundant alloys we find in karated metals are silver, copper and nickel. There are other metals in small and trace amounts but the average person has little knowledge of them.  Among the three metals, nickel is the toughest and also the hardest, followed by copper and then silver. Nickel is found in white gold. Because of its bleaching effect and when added to yellow gold even in small amounts, it has a significant effect on the colour of the metal.

Nickel aside, we are left with copper and silver. With these two alloys, it is possible to make an 18K alloy as hard as a 10K alloy. When you combine copper and silver in equal amounts and then add the alloy to gold, you end up with a karated metal that has a very hard quality. As you increase the silver content the metal tends to become softer (in 18K) and greener in colour. North Americans tend to favour 18K gold that has a slight greenish tint over the deep yellow colour favoured in Japan.

Let's look at some of the basic alloy percentages that are commonly used in the three karatages being discussed. In 18K we have two popular blends, the first favoured for colour by North Americans, and the second by Japan. The gold content is 75% in both. The silver content in the first sample is 16.1 % and copper content is 8.4%. The second sample has equal silver and a copper content of 12.5%.

In the samples of 14K we have a pale yellow, with 25% silver and 16% copper, and then we have the alloy percentages reversed and a dark yellow colour. In the 10K alloys, we find the one with the greatest popularity has an 11% silver, 40.3% copper and 6.5% zinc content. Zinc is added to reduce the oxidation during the melt.

If we compare sheer hardness of the various karatages, we will find that the second 18K alloy, the first 14K alloy and the 10K alloy will have similar wear characteristics. Where they will differ significantly is the resistance to stress corrosion.

The 10K alloy is very susceptible to stress corrosion. It tends to react with body acids in some people and they may have a reaction to their jewellery. It is for this reason that earring posts are commonly made in 14K. When the gold percentage reaches and exceeds 50% the reactivity of the metal is reduced and becomes stable. This is not to say that some people will not have a reaction to 14K (58.3%), it just means that the metal will not readily react with oxygen and body acids. Rarely, do we hear that someone has an adverse reaction to 18K. If this is the case, they should purchase jewellery in the Platinum family of metals.

When you look at making a decision based on wear characteristics alone, this is not enough difference to favour one karat over another. If you view the issue from the standpoint of which will give you the lowest reaction rate, the 18K wins hands down.

.......Gold is alloyed for a variety of reasons, and the ones to be most concerned about are the look and price.