Pure gold is designated as 24 karats in the karat system.
Karat measures the mass of pure gold in an alloy as parts out of 24. Thus, 18-karat gold contains 18 parts gold and six parts other metals, making it nominally 75% gold. A 22-karat alloy is about 91.7% gold, while 14-karat gold is about 58.3% gold.
The word “karat” in this context is distinct from “carat,” the unit used for gemstone mass. The 24-part scale is an older purity convention; modern hallmarks may also use millesimal fineness, such as 750 for 18-karat gold or 999 for very pure gold.
Although 24-karat gold is commonly called pure gold, absolutely impurity-free metal is difficult to produce. Commercial 24-karat products are often 99.9% or 99.99% fine. Lower-karat gold is deliberately alloyed because pure gold is soft and easily scratched; copper, silver, palladium, or other metals can improve strength and change its color.