The purity of gold is measured in karats (k). Pure gold is 24 karats and is very soft. Anything less than 24 karat gold is gold that has been alloyed with other metals for strength and hardness.

18 karat gold is 18/24 gold, which is 75% gold.

14 karat gold is 14/24 gold, which is 58.5% gold.

10 karat gold is 10/24 gold, which is 41.7% gold.

Yellow gold
Yellow gold is the classic and traditional choice for an engagement ring. Since pure gold is too soft to use in jewelry, it is alloyed with other metals, such as silver and copper. Yellow gold bands look good with warmer colored diamonds (color K-Z).
Pros
• Less expensive than platinum
• Will last a lifetime

Cons
• Polishing takes away some of the metal.
• Yellow gold makes a diamond make more yellow. However, a platinum setting will make this less apparent on a cooler colored diamond (D-J)