The alloy with gold content 18% is used to synthesize diamond. 18k is an alloy with gold content 18%, synthetic zirconia is a synthetic diamond, and 18k gold synthetic cubic zirconia chain card means that an alloy with gold content 18% is a synthetic diamond. Cubic zirconia, also known as Soviet union stone, is a kind of zirconia crystal. It can be synthesized by artificial methods and is widely used as a substitute for diamonds.