Pure gold shell. For some good old brands, customers often ask for a pure gold shell, which means that the shell is made of 999 pure gold, which is 99.9% pure. The simple substance (free form) of gold, commonly known as gold, is a popular precious metal, which has been used as currency, preservation and jewelry for many centuries. In nature, gold exists in the form of simple substance in gold nuggets or rocks, underground veins and alluvium.