Gold is used to make jewelry because of the attractiveness of its luster and because of its rarity. It also doesn't tarnish and doesn't react with air the way a metal like iron does. Gold is malleable, which means it can be beaten into very thin sheets. It's also ductile, which means it can be stretched out into thin wire as in the findings of earrings.
Twenty-four karat gold is pure gold and considered too soft to be made into jewelry. Therefore, gold is alloyed with other metals to make it stronger. The alloys used for jewelry are usually 16 or 18 karat gold.
When gold is alloyed with metals like nickel and silver it becomes white gold, which is a favorite material for wedding rings. White gold is often plated with rhodium to give it more shine. When gold is alloyed with copper, it's called rose gold. When it's alloyed with a bit less copper, it's yellow gold. Gold is blue gold when it's alloyed with indium or iron, purple when it's alloyed with aluminum, black when it's alloyed with cobalt and green when it's alloyed with silver alone.
Pieces of costume jewelry are also plated with the thinnest sheets of gold to enhance their appearance.