'pure' is probably the term that immediately comes to mind. As you are talking about gold then pure gold is referred to as 24 carat.
If it's "absolutely" pure then it would also be an element, no?
The term that can be used to describe a sample that is composed completely of gold and free of impurities is "pure gold." The sample can also be described as "100% gold" or "unadulterated gold."
