Why Not Pure Gold?
Pure gold is too soft to be worn as everyday jewelry.

Gold (Au) has a hardness and strength similar to lead (Pd), and as most can imagine, this would not stand up to much day to day wear and tear, and it will easily bend, dent, dink, scratch etc. For this, jewelers use a technique known as "alloying". In this process, other metals such as silver, copper and zinc are mixed with the fine gold to increase the mechanical properties; which make it harder and stronger, and giving it durability and wear resistance to day to day activities. Fine gold is used in the Middle and Far East as investment jewelry or for jewelry worn by a bride for her dowry.

The alloying technique also allows the jeweler to vary the color of the resulting alloy, giving a range of hues for yellow gold alloys and creating families of red, green and white gold alloys.