As a measurement of purity, 750 fine gold contains pure gold in 750 parts and other base metals in 250 parts. This measurement means that 750 fine gold is 75 percent gold. Although the purity of gold is measured by karats in the United States and the United Kingdom, there are many European countries using the decimal system to indicate the parts per thousandths of pure metal for silver, gold and platinum alloys.
The parts per thousand purity measurement system is referred to as millesimal fineness, or normal European stamping. The older karat system, sometimes spelled "carat," measures purity by fractions of 24. Gold alloys measured as 18 karat, for example, would represent the presence of pure gold in an amount of 75 percent. The equivalent millesimal fineness measurement for 18-karat gold would be 750.
The purest gold ever produced was in 1957 at the Perth Mint in Australia. The millesimal fineness measurement was 999.999, also referred to as six nines fine. In the U.S., the Federal Trade Commission requires that all gold jewelry sold must be at least 10 karat, 41.67 percent pure gold, and properly designated regarding its purity.
Because pure gold is soft and malleable, it is often alloyed with other base metals to improve its hardness. Gold also has the ability to withstand tarnishing and maintains its color and brightness over time. Gold pieces retrieved from ancient Egyptian tombs appear similar today to their original condition 4,000 years ago.