Pure Yellow Gold (fine gold) is softer than pure silver, but harder than tin. Its beauty and luster are unmatched by any alloyed gold (pure gold mixed with other metals). The extreme malleability, ductility, and softness of pure gold make it practically useless for jewelry applications. It is just too soft!

The addition of alloying elements (other metals) to gold are used to increase the toughness and hardness of the metal. While almost any metal can be alloyed (melted) with gold, only certain metals will not dramatically change the color or make the metal brittle. The addition of the metal like Indium (great as an alloy with Platinum), for instance, turns gold into a purple color and gives resulting gold the workability of glass. Any pressure and it shatters.

Over time, certain percentages of gold have become legally recognized "karats". The karat indicates the amount of gold as a percentage of the total:

24 karat is 24/24's gold or 100% gold

18 karat is 18/24's gold or 75% gold
(25% of the other metals are non-gold)

14 karat is 14/24's gold or 58.33% gold
(41.67% of the other metals are non-gold)